It's a bad habit that I've (mostly successfully) tried to stop myself as well. Growing up, I was struck by two realisations:

1) I had dismissed sarcastically (or worse) many people as idiots who I discovered later were right

2) I have very little capacity to determine whether someone is thinking two or three steps ahead of me, at least from a couple of forum posts

The pain of remembering multiple situations where I was 'that guy' is enough to keep me in check, mostly :)