Why so much focus on minuscule negativity instead of umpteen positive things?

I find it difficult to understand this part of human nature. We tend to focus too much on rarely seen negative part of a person or situation. We ignore all the positive things about the person or situation. Even we do not spare ourselves from this habit. Probably because we give in to the momentary feelings of the mind. Mind is irrational and we fail to understand it and the feelings that arise in it. Trying to put oneself in other person's shoes and reflecting on the situation may be helpful. Giving time for rationality to prevail and letting go off the momentary feelings would make us ignore small negative things and make positive things visible. But you should be alert enough that other person does not take advantage of you time and again. It is better to disconnect from such a person rather than being connected and losing peace of mind.  

Sometimes we have too much expectations from someone. If that person is not able to fulfill those expectations, we tend to think that he/she is mean and does not care. Instead we should try to understand his/her situation. If he/she really cares for you and is not able to fulfill some expectations, there would definitely be some or the other reason behind that.