Many reasons. Maybe not doing that thing will be bad inthe long run. Maybe you want the fruit of doing that thing, but cannot enjoy doing it right now, etc.

A popular example would be sports if you're not very fit. You would obviously have tangible health benefits if you did it, you may look more attractive, you may have more energy both physically and mentally. But since you're a couch potatoe sports is demanding, exhausting and sucks. Would you do more sports it would suck less or you might even find it enjoyable, but you don't and that's where you are.

This puts you in the weird spot of wanting a thing but not wanting to do what would get you there, even if the reasons you don't want to do it would vanish if you did it.

You can only really overcome this mentally, e.g. by priming yourself in certain ways, or by creating situations where you don't have a choice, because others rely on you, etc.