You're not sure what happened or when it happened for that matter , but your sex life if just plain boring. We hear you! It happens to the best of us. The two of you used to have so much fun in bed together, trying out fun sex positions and just overall having a good time in bed.
Even the "nicest" guy could be after sex. Guys have methods of trying to get laid without committing. These types get what they want and once they are satisfied, they are gone.