This is so simple, yet so true:

it occurred to me that there are two types of people we tend to spend too much money on. They are the people who we would spend any amount on, but they are happy with any gift and the people who will not be happy with any gift no matter how much money we spend.

This article goes on to say how we can stop spending money we don't have or more than we really wanted to, on both of these types of people.