This is partly due to the fact that a relationship's success is generally determined through its longevity, the most successful being "until death do us part." The very nature of a causal relationship goes against this premise.
While I'm sure there are a few unicorns out there, who have dated the same person casually for decades, that's not typically how a casual relationship plays out.
The premise was simple: Are we OK with how things are going in our relationship?
Are we finding ourselves really falling for the other person, and would we like to take things to the next level?
But our relationship didn't naturally fall into this comfortable groove.