Perhaps the lesson is for parents.
Do welcome your children back into your life.
Don't take for granted, the children who stand by you steadfastly.
Why is it never interpreted as a lesson for parents in how to value their children and be sure to validate their children's feelings?
Why did he not throw the party in honour of BOTH sons? Wouldn't it have been better for him to show an appreciation of the return of the younger and the steadfastness of the elder?
Why did he do something that would foreseeably divide his sons and promote adversity between them?
Why did he not run out into the field with the younger one exclaiming, "Your brother has returned and both my sons are here and well and safe. Let us kill a pig."
I'm thinking this story is to teach us 'how to' and 'how not to' be an effective parent.