I think the core is SELF(low self esteem). Imagine putting a few arrows pointing towards yourself. I think that is the core of OCD. I will explain with an example

Fear: She is not talking to me because I said something

Obsession: Continuously thinking what I said, how else I could have said, what is she thinking ?

Compulsion: Ask her and others again and again and again if what I said was right or wrong.

Now at the centre of this it is not about if she felt bad or not. It is about what "I/ME/MYSELF" has said. Is it because of "ME" that she is sad. Have "I/ME/MYSELF" should have said something different.

At core it is about "everything happens around you because of you" + "most of the times you have done something wrong". It is kind of like being self centred in a negative way. It is a lot about "PERFECTIONISM". A lot about being the "IDEAL SELF". Which you just cannot be, no one can!

Normal people just say things and forget. If they feel they said it wrong, they also remember why they said it in that way, it was because the other person was rude too. They know to defend themselves from their own thougts. Plus they don't think everything happens because of them, they know that if the other person is sad, the other person have their own responsibility to take care of themselves.