You are very right that your grandpa (and other T-doms) need to let people fail themselves or allow them to learn on their own. I guess in our mind, we're thinking, "Surely this person wants to be A+++ in everything just like I do, so I will show them EXACTLY how to do that." We're trying to show you our process or our method for reaching precision - because we assume that you would also want to do that.
Regarding your last sentence - it wasn't until I came here to TypoC actually, that I read something that simulatedworld (an ENTP) said that really helped me. He said something like, "INTP's should recognize that it's not necessary to WIN every single social situation - to "defeat" every foe or every task or every situation with 100% accuracy." It's just not necessary all the time. That's an eye-opener for INTP's, I think. Cuz our brains are wired to want to be "correct" every single time. So, we have to conciously remind ourselves (or be nicely reminded by someone else), "Hey, this particular situation isn't that big of a deal. An argument or a debate or being accurate is not really necessary in this situation."
EDIT: Think of a basketball player. He knows (statistically) that he cannot make every shot. But, being a perfectionist, he still WANTS to make every shot. Every time he misses, he is angry with himself because he feels he could have done better. Even though he knows he will miss some shots, in the game of basketball, the point of the game is to "make shots" - so when you miss, you feel like you failed. And it doesn't matter if you *almost* made the shot. Almost only counts in horse shoes, hand granades, and slow dancing.