Money ceases buying you happiness when you've bought everything you thought would make you happy.
Then you realize that you're just bringing in money to spend on nothing and nobody, because you're on top of the world and you're all alone. And you need to scrape the bottom of the barrel to find ridiculous things to buy and do because it is the only thing that can give you a small bit of euphoria.
So you spend your money on ways to keep you busy and keep your mind off the fact that you're all alone. Your friends can't keep up with you, and only gold diggers want you around and you just can't seem to fall in love with them because you know it isn't real. They think they have you fooled, but they don't.