Do not make assumptions.
It is a human tendency to make assumptions about everything. This usually arises from a fear to ask for clarification and ends in us trying to prove someone else wrong when we are the ones with misinformation. Making assumptions can be particularly detrimental in relationships, as it creates conflict and makes us feel let down and unhappy. The most common assumption in a relationship is that the other person knows you so well that they will always do what they want. When they don’t do what you assume they should do, you end up feeling hurt.
You can even make assumptions about yourself and overestimate or underestimate yourself. This can lead you to feel disappointed in yourself, and thus unhappy.