Human Tendency

Well your observation is very correct. We do tend to lose courtesy with the people we know for a long time. But then again it is a good thing cause if say you were to pull the chair for a girl on first ten dates it is very nice and caring on ur part but if you do that everytime for ur 2 year long girlfriend having dinner at ur home, it would would not only be tiring for you but a embarrasing for her too

Yes, I truly agree with this. This is because you never say thank you and you doesn't be courteous to yourself. And when you become close with him/her you will feel that he/she is also a part of your life. And moreover, you feel hurted when they get wounded, this is because of this.

Well yeah, but sometimes it's not always a bad thing. I would prefer people I know well to be more casual and frank with me as opposed to feeling like they have to walk on eggshells with me, and I have a different dynamic with different people, and it changes depending on the person. It's not a bad thing - it just shows that two people are comfortable with each other, and can be more real with each other without having to put on an image.