It's mainly because once I start, I can't stop and all of the bad and maybe bad feelings I've had manifest. It's because it simultaneously breaks down a dam and my heart. I don't deal with many messy situations in my life anymore. I've tried to hold it together a bit better than that.
I'm happy that way or at least that's what I think most of the time. So when I start crying, and I'm not talking about a random tear drop or crying at the television or movies (which I do often), I can't stop. All of my hurt bleeds out through tears; illogical and logical. It's a painful experience I cannot contain. I've remembered this in the back of my brain and that's part of why I don't cry so much any more; even when it's warranted.
But when I cried like I did tonight when I really should be sleeping, I remember exactly why I hate crying so very much.