I really like your post. Because it shows that often times when dumpers have no "real" reason to dump someone everything lies inside them. However not so many people take the time to look inside themselves and develop a more mature way to see relationships. Most likely you went from honeymoon to next stage and thought "things are not the same" as obviously they can't be the same after a long time with someone.
Had you had someone new coming to your life, you'd start becoming interested in him trying to see if that grass was greener. It might be, it might not. But I think now is the time when you are becoming fully aware that feelings come & go and it's not wise to always act on them.
Being said that, I can see why breaking up is always the thing to do. Really there is no other option as being in a relationship while half-hearted is quite hard to fake. But instead of jumping onto someone new, taking the time to dig inside oneself seems a more profitable way to achieve better results in the long run.
I wish you the best for both of you.