LPT: If you “under-promise” it’s easier to “over-deliver”.

When you promise to do more than someone asks for, even when you do exactly what they asked for, they’ll still be somewhat disappointed.

On the flip side, it’ll seem like you’ve gone above and beyond when you actually delivered exactly as requested.

Edit: This is handy for cases where you’re doing favours for other people. Stuff that doesn’t really matter to you more than the person’s opinion about you. Like some have said, it’s not exactly great for personal growth, challenging yourself, etc.