Feeling good for moments

[AnthroHeart]

I alternate between depressed and feeling very good.
The good feeling lasts only a few moments.
Is this a brain chemical that causes me to feel good?
It's usually when I see something that excites me.
But sometimes it happens for no reason at all that I can think of.
Just a few minutes ago I was happy for like 10 seconds.
Then was back to ordinary, feeling a bit down.

I know I can't be upbeat all the time, because that is manic.
And being manic isn't healthy either.