We all have quirks and small compulsive behaviors. Think of the things you do the same everyday and a certain way. They usually become what we call habits. Being agreeable can be developed as a habit as well.
One of my quirky compulsions is that I must set the volume control on any electronic device on an even number. Another one is to try to be nice by saying "thank you" or "please", you know, one of those first polite gestures you were taught when you first engaged with adults as a child.
When one witnesses someone being 'nice' we can easily identify it as such. There is no ambiguity. We know it when we see it. It costs absolutely nothing except an allowance of pleasantry that we can spread around us. Recently, a report came out of a Dairy Queen manager, Joey Prusak. He did something nice and received national attention for it. He was shocked one day that a customer plucked someone else's $20 bill off the floor and slipped it into her purse. She then came up to the counter to order, but Prusak refused to serve her unless she returned the money to the visually impaired customer who was unaware that he dropped the bill. The woman refused (not nice), so Prusak gave, out of his own pocket, the $20 to the gentleman. Nineteen year old Prusak said, "I did it without even really thinking about it...Ninety-nine out of 100 people would've done the same thing as me." (Now that's nice) It can be seemingly a small thing but it can be a genuine, heart-felt compulsion-something we do all the time, everyday, because it can create an awareness that there is someone other than ourselves at that moment...someone who might need you to just be nice.