I learned that I have inherent biases. We all do, they come from our development and, to a surprising extent, from our genes. It's only once you recognise these biases that you can control them. You realise that many issues don't actually have a single reasonable answer; it depends on what you accept as basic truths, and especially when it comes to things like morality, these can ultimately be baseless, or highly malleable. At the very least, you realise that different people can sincerely come to different conclusions. In a word, you learn tolerance for all people.

Almost all, anyway. I still have zero tolerance for liars; people who do not come to their world view sincerely, but rather by a process of lying to themselves for whatever reason, and to maintain the facade they lie to you, too.