A few simple steps to improve your relationships can help make sure you are the kind of friend you would wish to have yourself.

Human beings are notoriously inept at recognising our failings. We may complain about another person's arrogance, ignorance or stupidity, without ever considering the enormous flaws that they may find in our characters.

This blind spot will be evident in each of our friendships. Without ever meaning to do harm, our thoughtless actions hurt the people we love best. I'm talking about casual cruelty rather than deliberate unkindness – but the consequences of these slip-ups are damaging.

While writing my recent book on the science of social connection, I discovered that "ambivalent relationships" – people who blow both hot and cold – can cause even more harm to wellbeing than purely spiteful figures who are predictably unpleasant. Fortunately, research findings can help us develop simple-yet-powerful strategies to identify our worst habits and mitigate their damage. Here are my five favourite lessons to avoid being an accidental frenemy.

Be consistent

No one likes to be kept in a state of uncertainty – a fact that can be seen in people's responses to physical pain. Archy de Berker at the UCL Institute of Neurology in the UK, and his colleagues, asked people to play a computer game that delivered a mild electric shock whenever they found a snake hiding under a virtual rock. To examine the effects of uncertainty on the stress response, the researchers varied how likely it would be that a rock would be hiding a snake across the course of the experiment, and measured the physiological signs of anxiety – such as sweat and pupil dilation. 

Quite surprisingly, the participants tended to show a more pronounced stress response when there was only a 50% chance of being shocked, compared with scenarios when they knew for certain that the pain was coming. 

Fairweather behaviour may put the people around us in a similar state of anticipation. In studies investigating unpredictable friendships, scientists ask participants to imagine going to a friend for advice, understanding or a favour. They ask them to respond to the following questions, on a scale of one (not at all) to six (extremely):

• How helpful is your connection?

• How upsetting is your connection?

Anyone who elicits two or more on both questions is considered an "ambivalent connection" – and the inherent doubt about their reaction can be a serious source of stress. In one study, simply knowing that their ambivalent friends were sitting in the next room was enough to raise participants' blood pressure.

We may not always be able to provide the support our friends need, but we can try to be a little more reliable in our responses. We might learn to manage our bad moods better, for example, so that we do not lash out if our friends approach us at the wrong moment – rather than placing them at the whim of our emotional weather.

Avoid the illusion of transparency

Each of us is trapped in our own minds, but we overestimate how well others can read our emotional state – a phenomenon that is sometimes known as the illusion of transparency.

This may be evident at job interviews: we assume that our nerves are written all over our face – but the anxious feelings are often far more difficult to discern than we realise. This common cognitive error may also prevent us from sharing our appreciation of others, giving them the impression that they are neglected and undervalued.

Amit Kumar, at the University of Texas at Austin, and Nicholas Epley, at the University of Chicago, asked groups of participants to write letters of gratitude to important people in their lives. Using surveys to measure the letter writers' expectations and the recipients' actual reactions, the researchers found that people consistently underestimated how surprised the other person would be to receive their kind words, and how good it would make them feel. They assumed that the other person already knew how grateful they were.

It is, of course, possible that our body language will convey our warmth and appreciation to others, but we can't rely on that fact – meaning that we would often do far better to express those feelings in words.

Validate others' feelings (but nudge them to consider new perspectives)

When someone is going through a hard time, they will often naturally seek understanding from others. An empathetic response can validate their feelings, which eases some of their stress. A toxic friend may be highly dismissive or judgemental about your feelings – resulting in a sense of rejection that only adds to the person's emotional burden.

Just because we feel sympathy for someone's pain does not mean that we have to wholeheartedly agree with their interpretation of the situation, however. The most effective offerings of emotional support will often include encouragement or advice that helps them to see their problems from a new perspective. 

Indeed, a growing body of psychological research suggests that simply encouraging someone to vent, without any attempt to reframe their problems, may only encourage rumination and amplify their distress in the long term. This vulture-like tendency to feed off another's emotions – without helping them to change their situation – is therefore another form of toxicity in friendships. 

A constructive conversation needs plenty of sensitivity and tact, but a paper by Ethan Kross at the University of Michigan and colleagues offers a few questions that may help someone to view their problems through a wider lens, including:

• Looking at the situation, could you tell me why this event was stressful to you?

• Have you learned anything from this experience, and if so, would you mind sharing it with me? 

• In the grand scheme of things, if you look at the "big picture", does that help you make sense of this experi­ence? Why or why not?

After considering these different points of view, participants in the study tended to feel greater closure about a painful event, compared with those who had recounted the concrete details of the situation and the feelings it had produced.

Celebrate each other's successes (and practice confelicity)

Empathy is equally important in the sharing of positive emotions. Compassion, which derives from the Latin for "shared pain", is well accepted as the foundation of friendship, but the importance of "confelicity", deriving from the Latin for "shared happiness", is much less well-known. 

This neglect has been embedded in the scientific research. When Shelly Gable at the University of California, Santa Barbara and Harry Reis at the University of Rochester, New York, surveyed the psychology literature in 2010, for instance, they found that the number of published articles focusing on negative life events outnumbered those focusing on positive life events by more than seven to one.

This is now changing, with multiple studies revealing that our conversations about good news can be just as important as compassion for the development and maintenance of healthy relationships. A supportive friend should respond actively and constructively – asking for more information, discussing the implications, and expressing their own joy or pride. Many people, however, respond too passively – by quickly changing the subject, for example – while a few are actively destructive, making comments that attempt to minimise the events' significance.

With the distractions of daily life, we may forget to give these moments the attention they deserve, but if we want to be a good friend, we should take the time and effort to celebrate our friends' successes – however small or large they may be.

We might also think more carefully about the way we share our own happiness. We may worry that we'll seem boastful or arrogant, and so choose to keep our successes a secret, but this strategy can backfire badly, according to a series of experiments by Annabelle Roberts at the University of Texas at Austin, Emma Levine at the University of Chicago, and Övül Sezer at Cornell University. In general, they found people tend to be highly offended when we hide information like job promotions from the people around us. They see the behaviour as paternalistic, establishing froideur in place of warmth and connection. 

Be the first to say sorry 

Everyone makes mistakes, but few will apologise freely, allowing resentment to fester in our social bonds long after the offence has been committed.

The psychological research suggests that there are four main obstacles to apologising effectively: we don't appreciate the harm we've caused; we assume that the act of apologising itself will be too painful and shaming; we believe that the apology will do little to repair the relationship; and finally, we may simply not understand what constitutes a good apology – so we fail to say the words that will be necessary for healing. 

The first point clearly depends on the details of the disagreement. But the next two concerns – like so many of our assumptions about relationships – are largely unfounded, and therefore present unnecessary barriers to social connection. In general, people feel relief at making amends for their wrongdoings, and we may be better able to rebuild the broken bridges than we expect – provided our apology is given in the right way. 

To ensure that your apology is effective, you should give the other person plenty of time to express their hurt at what has occurred. You should then acknowledge responsibility for the offence, express (genuine) regret or sadness, offer to repair the damage and explain how you will avoid making a similar mistake again.

Every relationship will have its ups and downs: that is the nature of human behaviour and the complexity of our social lives. By applying these five simple tips for stronger connection, however, you can easily avoid the most common errors, ensuring that you are the kind of friend you would wish to have yourself.