Because they get tired/sick and have problems too. From my experience guys don't like to share their problems, least of all to a girl they like. I think they feel it makes them look weak or that they can't handle things the way they're supposed to or something. In a way it makes sense. They don't want to tarnish the impression that you have of them. Even though a lot of girls want guys to talk to them because we want to help. Even if it's just to offer a comforting hug or lend an ear to listen.
Must be the guy(s) you are seeing. I am far from moody, and having been raised in a high-stress household (my mom's business would make her have constant mood swings), I have learned to keep emotions/stress to myself, because nobody wants to deal with your problems.
In fact, of all the qualities I love about my girlfriend, this is one of those few things that I complain about her.