Obligation VS Courtesy

Obligations VS. Courtesy, are these two of the same meaning? Actually if we look these two words up in the dictionary they are totally different in meaning. 

Obligation: Something by which a person is bound or obliged to do certain things, which arises out of a sense of duty or result from custom, law and etc.

Courtesy: A courteous, respectful or considerate act or expression. 

I believe between these two, some people are abusing their rights and it really get on my nerves. So what exactly is the issue here? Let's do a recap shall we. 

First and foremost, I did not do anything what-so-ever that was backstabbing this person. But I have no idea why whenever this person comes to work will show that sour face as if we or I owe this person a lot of money. I mean da fuck this is seriously and ATTITUDE problem. If you have issues just say it don't show the fucking face to us. Once or twice is ok but this has been something that has been prolonged and it gets on the nerves.

This person mentioned we have done something that made 'the person' mad but never clarified what. But one questioned asked ticked me off. It was about lunch and that we did not ask the person to join. I explain that I went off with friends but the replied I got was I listen first. I was like wtf.

To me I am not obliged to ask you out for lunch it was basic courtesy to ask you. I am not going to always ask you to join. FYI, when you go out having lunch with your friends you basically shun us aside and we did not say a single thing and we didn't mind. Everyone has their own lives for goodness sake. We are not obligated to keep asking you out for lunch everytime. 

And small issues which can be solved EASILY can tick this person off....I mean honestly it is really annoying when you are off on study leave and questions like these are being asked and last message you get was we make u angry... honestly....I don't find it intriguing nor amusing I think it is down right childish and inconsiderate...