Money, mostly.
People who are driven only by money in life are pretty sad people, I think.
In fact, if you only measured success in monetary terms, you would likely live a pretty crappy life.
Indeed , i have a friend that lives only for money and don't even wanted to have a family , and Yes he is very sad even if how he pretend to be happy being rich ? yet we can sense and see in His eyes the sadness of being lonely.