I'm not entirely sure hope helps, since Tack's observation quite literally supports my own point of view in my own biased ways.
In a perfect, contractual world, you stop being relevant the moment you execute your obligations. But, people know they can use you for something else. Perhaps some emotional support? Perhaps, a different "contract", or as would be more commonly known to people, favors and tasks. They can't quite throw you away yet.
They stop relying on you provided you fulfill your purpose of friendship/labor/whatever, or when they find a better substitute.
As for intent... That's 'nother whole can of worms.
There is no such thing as unconditional love.By that extension, my biases include the concept of true love in it. People tend to fall in love because you're who you are, in that very second. Given that you change for a bit, they might try to nudge you back into that path. "Don't change. Don't ever change."
But are you really in love with that person? Or are you just simply in love with the memory of that person, or who they once were? It's hard to say.
Divorce statistics say that commitment is a major issue (73%), also Unrealistic Expectations (43%). My biases tell me that part of said unrealistic expectations involve how the person actually lives their lives beyond just dating and etc. This changes your point of view, makes you question your love for said person, and bring up divorce when it comes to it.
Considering my parents just up and left me (and my brothers) alone due to matters of finance, my view is really screwed.