It's not that easy. Social contact theory can work when there's a relatively objective standard like "physical violence" but you'll often believe that the people you disagree with are being intolerant, and they'll believe that you're being intolerant. If the general rule is "I'll only tolerate people if I'm convinced that they're tolerant" then very soon no one will be tolerating anyone else.
With that said, I don't think there's a "paradox of tolerance" simply because tolerance is hard. The people talking about a paradox want to get credit for "tolerating" just the people they don't mind having around, but you have to tolerate the people you hate, the people you think are a threat to you. Otherwise you're not tolerant.