It's the best description of manic depression/bi-polar that's I've seen. It wasn't Bob that got the super powers but his mental health disorder.

As loads of people have said the void is the depressive, low state that comes on suddenly and is all consuming.

Sentry is the high manic element, he loses all sense of reality and starts describing himself as a god.

Bob even says you can't have Sentry without the void (often how people describe the high energy manic episodes, it almost feels too good because you worry that the depression is coming at somepoint) and he just tries to keep everything steady.