Paul, a Pharisee, knew very well what he was talking about when he wrote “if I have all faith so as to move mountains but do not have love, I am nothing.” The Pharisees, the elder son in today's parable, had faith. No question. They knew the Law and they did everything exactly so...but they did it purely for the reward they would receive. They did not have love, not even for the Father from whom they would receive everything. It was purely service, a this-for-that relationship. Certainly they did not love their “younger brother” - the nations, who weren't quite so attentive to the Law or to God at all.
Most of us are a mix of the older son and the younger. We're running off, returning, then turning around to squint at those who aren't quite as fast as we are. But all the same, we remember what the Pharisees did not – that we did run off ourselves, that God pardoned our sin no less than that of those who are still on the return journey.