Really don't know why.

I've had problems in the past, this is so very true. But life's been looking up for me, getting better. I've got an 'in' to a good repitable college, I've got really good friends who make me laugh, I'm in fun clubs at school, and yet I'm still getting randomly depressed.
I don't know why and I know it doesn't make any sense, but it's true. I'll be happy, laughing with my friends, then I get real quiet and stuff. At home it's the same way too, my parents and I are joking around, laughing like mad men, and then I stop. I usually go to get a drink and when I sit down again I don't feel like laughing, smiling, joking around, doing anything but thinking. Then all I can think about is WHY I don't wanna do anything but think.
I've got a good life, why am I always so depressed? Why can't I be like everyone else, happy with good reasons, mad with good reasons, sad with good reasons. Why can't I have REASONS for my crazy weird emotions? WHY am I not like every other NORMAL 16 year old girl?