The people in the blueish (top graph) group, won the lottery 2.5 months in. The people in the red dotted group, are inmates, that were sent on deathrow.
Note, that both groups had similar chemical levels before the six month and will have after, assuming they do possess a healthy brain. The reason is that a healthy brain, is a balanced brain.
Now, what could be the point of all the happy chemicals if none of it lasts?
You were born! YAY! Let's follow that graph to the first junction. Most will take the path upwards. They learn to not crap their diapers. YAY! ...the path downwards? Maybe you were born a quadriplegic and have no control over you body, so diapers for the rest of your life :< . Let's look at the next junction! Most people take the upwards path again, they learn to speak. YAY!
WAIT, how come some of our quadriplegics could even take another junction down? I mean, they can't move right? Well, some of them will learn to think and become aware that they are quadriplegics. The others will drool in bliss.
Don't feel bad though! Remember the other graph I made up? At each junction, the brain chemicals might make some feel happy, and make some feel sad, but the brain will adept to it's new surrounding and no matter which branch you end up on, you won't be happier or sadder than anywhere else if you possess a healthy brain, because a healthy brain, is a balanced brain.
Note, that while you might got lucky and learned not to crap your pants, learned to make memories, to walk, to talk, if you get old enough, you'll end up in diapers again! Unable to walk or even remember the memories you made.
Are we doomed?
Nope! In order to climb the branches, you had to learn and cultivate new abilities, by exercising virtues. The journey is the destination. Our brain chemicals guide us towards challenges and excitement, but it is not about material pleasure. It is about the opportunity to be excellent.
Even if you became god emperor of the world, with a harem consisting of all female members of your species, your brain would start to balance on that branch and show you new and exciting ways to grow, if you so choose to open your mind and heart. (Why limit yourself to females, or your species?)
Theory of purpose:"The brain relates to the universe, which the brain is part of, as to be our greatest teacher."