Why are people so sensitive about having their grammar corrected?

Personally, I like to speak the language correctly. I'm all for linguistic diversity, but, let's face it, grammar and spelling are going down the tubes. I certainly don't mind if someone points out a grammatical error I make; I figure they've done me a favor since I take care to speak the language correctly. My reaction is roughly equivalent to when someone tells me I've got spinach in my teeth. But it seems I am in the minority on this one. Most people whose grammar I've corrected, with the exception of a few family members and close friends, get offended and act as if I personally insulted their mother. All hyperboles aside, why is this? What is your experience with this?