I think Satya is right. Humans need affection. Look at the studies of old people getting depression from being ignored and invalidated. I try to compliment when I see something worth complimenting. I may compliment too much, but I don't really care. I don't think that it does any good for a compliment to stay in my head. You never know when the right words can make someone's day.
There are people on here who've totally made my day by saying something nice when I was really feeling lousy. Telling me nothing, or something negative wouldn't have helped. I also think complimenting is a form of being genuine.
Constructive criticism is helpful, but criticism just to criticize is not. I am thinking of artists in particular. How many people have stopped doing art because as a child, someone told them that they were no good at it? I've heard lots of stories like that.
Also, criticism that is meant to demean, or to make the other person feel better about themselves by demeaning you or making you inferior is just the wrong way to go about things. Criticism just to make someone feel bad is a bunch of b.s.
Constructive criticism on the other hand is valuable. For example, let's say you are a teacher and are grading a kid's paper. Let's say this paper is horrible. Do you simply mark it an F, and return it to the child with the note: HORRIBLE! Or do you point out things that could make it better? You could still give them an F, but tell them how to write a better one, and if they do the work, you might adjust the grade? That's constructive criticism - criticizing truly with an intent to help, not an intent to injure.