In my life, there have been people that I was convinced would be around forever, and yet, somehow they managed to drift away after a couple of years. Likewise there have been people who have begun as casual acquaintances but become more important with each passing year.
Acceptance is supposed to be a good thing - Grant me the serenity to accept the things I cannot change. Also compromise, as every couples therapist will tell you. But the cost was high - the damping of expectation, the dwindling of spirit, the resignation that comes to replace enthusiasm, the cynicism that supplants hope. The mouldering that goes unnoticed and unchecked.
I studied psychology for a couple of years as a personal hobby, so you start learning about people and listening to your intuition, like when you you're feeling that people are not being entirely straight with you.