I think one of the problems here is that we, as humans, tend to always want more than what we have. No matter how well we are off, there is always something to improve upon. Some people get caught in the endless loop of misery, when they want to achieve something in their life, they achieve it, then immediately they start to want to achieve something else... They are never satisfied, never happy. It is a good trait, to want to move forward, but it can be pretty damning, if you never enjoy where you are at the moment.
A person that decides to devote their life to raising kids, can be miserable because of the lost career opportunities. A person choosing to focus on the career instead, can be miserable because of lack of kids or meaningful interaction with them.
Ultimately, a person just needs to determine what they want in life and move on that path, always striving for better, but never complaining about the present either. It doesn't take that much to be happy, as long as the person does what they think is right and enjoys what they already have.
Then, again, everyone has unique mentality, so maybe it isn't applicable to everyone.