When do you know when someone is just being too sensitive and it's not that you're not being thoughtless?

What's the definition of "too sensitive"? We're all very sensitive about one thing at least. None of us should act in a thoughtless manner and then dismiss the other person's sensibilities by saying "Oh, you're too sensitive." (I'm not saying that's what you do, by the way, I mean in general.) If someone is clearly put out or distressed by something we've said or done, then it's good manners to avoid that topic in future with that person.