In this age of social media, we are living in a phenomenon of "positive feedback". When a person posts in facebook, those "likes" give him or her a sense of approval. In search for this pleasanf feeling again, in the future, that person will be motivated to post again. I think that is the psychology behind it. Scientifically speaking, if you wanna know the biology behind it, I've read that once a person feels that sense of approval from the likes and comments, Endorphins, a chemical in the body is being released. This is the "happy hormone" responsible for good and pleasant mood. This is what people get from likes.