I really wish I didn't care, but I can't help it.
When I hear things like : "between you and I", which is the sort of error I hear all the time, it makes me grind my teeth.
The greengrocers' apostrophe annoys me, and I've been known to rub it out if it is in chalk.
When people talk about something being 'decimated', when far more than 10% has been destroyed, I want to correct them.
It is our language after all, and I feel a sense of ownership, so when people hurt it, I get very cross.
I'm happy with new words though, and new usage of old words, like mouse. I'm happy for us to absorb foreign words too; they brighten up our language.
I don't think it is anything to do with the class system, but I suppose it is true that those of us who went to an old fashioned grammar school generally get it right.