It’s ok, but you also need to understand that everyone else is doing the same thing. You shouldn’t be surprised when someone else does the same thing you are doing, and if you expect other people to treat you better than others, you are in for disappointment.
Most of us treat our friends better than strangers. The closer we are, the more we want to spend time with them and give to them. When your close friend invites someone else to something like this -- long event, lots of time together, probably expensive -- it's understandable to ask whether they consider that person a better friend than you. You are probably wrong to jump to that as a conclusion, according to the article and my experience.
This person's reaction has nothing to do with the third person getting invited, and everything to do with them _not_ getting invited and what it says about their relationship with the gift giver.
Your statements seem orthogonal to what the author is actually thinking and concerned about.
