why do some people hear me state something with conviction and take it as a personal attack?

Them: "So I'm not sure what I think about X."

Me: "I really believe in X, because of A, B, and C!"

Them: "Oh, I'm sorry, I didn't mean to offend you!"

I'm sure it's something about the tone I'm using, but I thought it was distinctly different from the one I use when I'm getting upset. This is just me getting excited - speaking a little louder than usual, maybe, but I'm not snapping or yelling. Or is it just that I rarely ever actually say anything that firmly, that they think I've got to be upset about something?