Who am I mad at?

I have heard over an over that you teach people how to treat you.  I often wonder about those that are rude and treat people so unkindly, what they must be thinking.  Why in fact do they feel that it is alright to belittle or degrade someone?  What could have happened in their life to give them that sense of entitlement?  Do I feel sorry for the cashier that was rude or the family member who does not know how to mind their tongue?

If someone does treat me poorly and I don't do anything about it, then who should I be angry with?  Them or me?  Didn't I allow their bad behavior to hurt me?  After thinking about this over the years (it seems some good answers come with age), I have come to realize, rather than lower myself by being confrontational, it is better to disregard their behavior.  Some would say (and have) that this is passive/aggressive.  However, when in the past I have confronted someone for their inability to be civil, later I would revisit the scene over and over in my mind.  Wishing I had said this or that, or that I had not allowed myself to get caught up in the immaturity of the situation, until I was exhausted.  So, I could be wrong but I choose to be nice, no matter what. Maybe.