I think the worst thing about the whole "needing a reason" bit is when you know the person is asking in an effort to help. I've just about got my partner to the point where he'll accept that sometimes my moods have no rational explanation at all, and that sometimes all I want to do is vent the bad mood, rather than have the problem solved. But he still has this big need to "help" somehow, and it makes me feel worse because there isn't anything he can actually do to help me.