Entitlement

Perhaps it's a good thing...
That I haven't felt the need to use this forum to vent about my life in the past (OMG) 3 years.

How is it that we come to feel entitled to THINGS?
It's one thing to earn something.  Compensation is a just and right thing.  You are entitled to be paid for your work.
     Wife just had a birthday.  My mother and she have not been close.  Ever.  Wife has never felt accepted by her.  Several months ago, mom found out that Wife is no longer Christian, and was emotionally hurt by it.  Likewise, Wife was hurt at the insinuation.  But now...
     Wife just had a birthday and got a check from Mom.  My dad died last Nov, and now Wife is convinced that it was he that gave her a goodly sized check for her birthday, because this year, the check was less than it's been in the past.  However, Wife doesn't see that it's still a 3 digit check, just that it's smaller, and she sees it as a slight.  She's overlooking the fact that it's money that we didn't have a week ago, that it's money that we were NOT counting on (at least I wasn't, but evidently she WAS).  Now Wife is more bitter than ever at mom, and isn't going to send any thank you card or call.  To top it off, she chewed ME out for thanking her in my weekly phone call to mom.
     How is it that a person comes to believe that they are entitled to something?  Is it a history and repetition that breeds it?  Is it just my detachment that sees this as a problem? I'm ready to pare down my life, get rid of Things. Maybe it's watching all the students come and go for the summer; some are selling apartment furniture, some are bemoaning 4 years of accumulated shoes.  All are faced with having to constrain their lives and possessions to a single car-load.
Priorities.  Which memories to keep, which to let go.
Which THINGS are important?