"Successful Relationships"

I have contemplated this phrase today "successful relationships"...and I have come to the conclusion that the phrase in itself does not make any sense whatsoever.

What defines a successful relationship? Is it the length? Does the longer it last affect it's worth and it's status? Or is it whether the relationship ended on good terms? Or is a successful relationship a relationship that is ended by death?

Can anyone tell me what a successful relationship is? Is it the enriching of one anothers lives with culture and new experiences? Is it walking away from someone, happy to have known them, having grown wise through the relationship? Is a relationships worth dependent on the amount of tears shed over it?

If you haven't been married to someone until death does part, have you never had a successful (non-plutonic) relationship? How do you put a label on whether or not your relationship is successful?

My lackluster conclusion is this -- we cannot label relationships with the term "successful", because relationships are active, changing, and moving things, akin to life itself.