Why do some people get offended SO easily?

Do they go looking for offense?
I have a younger sister who I virtually never talk to. When we do talk, she does all the talking, and it’s all about her, and if I trying to contribute in some way she just kind of says, “Mmmm,” then turns the light back on herself.
And I have to be REALLY careful of what I say because she gets offended at the drop of a hat. Either she takes it wrong, or simply didn’t understand, either way then she’s indignantly righteous, angry, and that ends the conversation.

So when we DO talk I’m very quiet, stilted and cautious, which is not conducive to a warm relationship.

Needless to say, I can’t even fathom what she’d have to say if I insisted on monopolizing the whole conversation and making it about myself. I guess there would be hell to pay for my selfishness.

Why does this happen?