If you don't try and you fail, it might be less painful than if you try hard and still fail.
You can think "I could still do it, if I really wanted to." It protects your time, and your ego.
This can become a real problem if you are trying to learn something that can only be done with full mental investment. Mathematics is like this. It requires a kind of intellectual vulnerability.
No one wants to feel stupid. Even just to ourselves.
@futurebird