Why do I always end up feeling horrible?

I have friends, family, and a girlfriend. I spend time with my friends practically every day, I chat with my girlfriend on skype every day and spend time with her when I can. I get to do things that I love such as play tennis and video games and just chill out during the hot summer days. But despite all of this I often feel hopeless and painful and think of suicide. It's not like I feel like this all of the time, I can go out and have fun and laugh and feel really happy, even high on life, but these depressive feelings always come back at least on a weekly basis, more oftenly every other day. Is this just general depression or what?