Are you the sort of person who just loves correcting other people's grammar? Are you sure that you're doing it right? Some things that people have been taught are rules of English grammar are really not rules at all—and some of them are flat-out wrong.
There's actually a word for this phenomenon: hypercorrection. It's what happens when people learn that something that isn't a rule is a rule.
Now there are plenty of reasons for people to learn about proper English grammar; it can make you a more confident communicator and help you understand the way the language has evolved. But sometimes, when people correct other folks' grammar in a non-education, non-copyediting situation, they're not being helpful; they're asserting their perceived linguistic superiority. And while some who proudly wear the badge of "grammar Nazi" or "grammar police" see themselves as defenders of the language, they're not really enforcing grammatical rules; they're reinforcing personal peeves.