Friendships and choices

I consider myself a kind-hearted person. I'm flexible and don't get upset often with my friends. However, there is a person in my life who has always known how to push my button. Actually I should say why do I allow this person to push my button. I know i have choices in life. I know this person can get under my skin, but yet, I always find a way to take whatever they say and take offense to it. How do I get beyond this? I don't like this aspect of me. I want to change. How do I do that? I don't have an answer for this problem in my life. If you have any ideas, I'd love to hear them.