You've been a really happy and loving couple once, but things started to go downhill. There isn't an exact reason for this; the love and "fire" from the relationship eventually fade out. But you still love him, and he still loves you as well.

"...I think it's time for us to part ways," He said, with a small frown and a melancholic look in his tired eyes.

He didn't truly wish to leave you, but things between you two were just... different.