Earning money they need to live is one of the most important reasons for people who work, but some other reasons do exist, at least it seems to me. People also work because they enjoy doing what they like, they like to work for a goal in a group, and they love the sense of success.
Almost everyone has some actions that are what he or she most likely to do. When they put most of their time on the things they love, they will feel they are the happiest guys on the earth. For example, some of the young today look forward to working as computer engineers, just because they love the moment to spend in front of the screen, the moment to put their tips on the keyboard, and the moment to solve a problem of it. To them the click of the mouth is a melody composed by themselves.
Some people insist on going to office everyday, although the Internet has made it possible for them to work at home. To them, they cannot be adapted to work at home without colleagues around them. To them, the feeling of a group is important. If they are separated from the working group, they will feel some lonely and sometimes may think that they were deserved by this world. So they will never give up the opportunity to work with others.
The last reason but never the least is that most of people like the feeling of success in the competition of modern society. For example, billionaires who have enough money to live a luxury life always see money in the check as a code of success. They works day after night, and only want to increase the number of the code, even though they know that the money they have is much more than they can use in their life, but they persist in working as if they are playing a race game.
From the reasons and examples above we can see that we work for money, but we do not work for money only. I believe that people who enjoy the work is happier than those who tolerate it only for the money.