Fear of failure is the usual reason - either because there's a risk of losing something you have or just being ridiculed by others.
But there's also fear of success. We often have subconscious beliefs that prevent us from succeeding.
For instance, if your goal was to make more money by negotiating much better conditions with your boss, aside from the fear of having to approach your boss, you might also have an underlying belief that "money is evil" or that "all rich people have cheated their way to success" or that "money can change you for the worse".
Or if your goal is to lose a lot of weight then you might think "slim people are so shallow" or "I don't have money to replace my entire wardrobe" or "men would treat me like a sex object and I don't want that".