A vow freely made carries a deadline, and the offering behind it must be consumed while it is fresh, not hoarded against some later day. A promise you keep putting off rots. The whole point of binding yourself with a vow is that it forces action now, in the daylight, before your resolve cools and you talk yourself out of it.
What a man commits to voluntarily reveals him more than anything demanded of him. The freely chosen burden is the one that shapes a life, because nobody made him take it up. Fulfill it promptly and it strengthens you. Let it linger and it becomes a debt that quietly eats away at your sense of who you are.