Which you?
"I'll do it."
You are pretty sure when you say this to someone, or to yourself, that when the time comes, as you said, "you'll do it".
But it's not you who will do it. At least, not the present you who is saying this. But the future you.
The future you may be tired or busy or may not just feel like it. But the present you is deciding for him.
Just like you won't put your partner on the hook without asking her first, it makes sense to discuss with your future self before committing them to something.
Having a trusting relationship with our future you is one of the best gifts we can give ourselves. After all, the future you soon becomes the present you.