There are some people who have a lot of crushes because they do not have someone to love just yet. When you are in a happy relationship, you may not become too attracted to other people because all of your attention is with the person that you love.
Having a crush might be interpreted by people as at least they still have feelings for other people no matter how serious it may or may not be. A lot of people just have happy crushes that are meant to inspire them to go to work or to go to school but they have no plans on acting on those happy crushes.