If there was no "bright side" to life, then why can people be happy?
If life wasn't good, even for a bit, if we had no reward system to say "Life is worth living" than we'd all die out, self preservation wouldn't be a factor and our species would of certainly not evolved for millions of years to this point today. What is the "bright side", well I believe in relative perceptions, in this regard I believe what is "good" and what is "bad" is dependent on the observer, not the object. For example, a song, without a living observer the song is nothing, uninterpreted and doesn't exist of any form of significance, however to an observer who likes the song the song is beautiful, and to an observer who hates the song the song is terrible. This is true for all forms of [things], including whether something is good or bad, the universe doesn't have a list of "this is going to be bad, this is going to be good", no HUMANS determine what is bad and what is good, and thus literally humans determine the bright side of life.
Essentially, is there no bright side? Depends who you ask. I believe, that in all logical equivalence there must, theoretically, be a bright side but my bright side is a bit more scientifically rigorous. The bright side of life, is the part in which you feel good, the part in which your body releases feel good hormones which make you happy. Your body releases these hormones in many circumstances, listening to a good song, being with the person you love, achieving a goal, etc. Your body releases these feel good chemicals so you can have a way of doing what is best for your body, including not hurting yourself which gives feel bad hormones and pain. I don't think it'd be theoretically possible for a species to exist without this sort of reward system that's put in place, or without a reward system at all, I think if there was no reward system and there was no consequence system than we'd all of died out thousands and thousands of years ago.
So, yeah, there is a good side to life. If you define it metaphorically than you determine what the good side is, if you define it scientifically than it's determined by your body.