Why are some friends on face book so stingy with their likes or sharing of your posts?

. I only have a few friends who like my posts on a regular basis. I’m always giving my friends likes or re-post their posts if I like what they say. “But they always ask for likes and say “if you like or agree with this re- post it”you would think that people who are so hot up for post- likes would like what you have to say too. But they seldom give likes and always ask for them. Why do you think this is so? I always give them the benefit of the doubt but still wonder why. Thanks