Parable of the Lost Son: What was Jesus trying to teach?

That if you are doing good things you should be doing them because you are truly a good person, not just to please God. Therefore you would NOT be jealous when you're brother (who was lost to God) found him and was praised. You would be just as happy as everyone else. That's my interpretation and I believe it to be the TRUE meaning. I've heard pple say I am wrong, but to me that is universal.