I've always thought about relationships this way. If I can't see myself with this person for the rest of my life, I'm not wasting any more time than I need to with them. Simple as that. I have far more important things to worry about right now.
I think it all started when I stopped liking him. I still loved him, but, I didn't like him anymore. U knew my mind was preparing my heart for that moment - the one I would say 'I'm done' and really meant it.