Because you've been in the same routine forever. Comfort is the enemy. In all aspects of life. Video game designers learned this ages ago. People need goals and aspirations, and this includes in a relationship. If you are not growing, you're dieing.
hardly anyone can maintain the "newness" of a relationship forever
I don't know, but it sucks...
Everything Has An Expiration Date
when you first meet them you don't know them, you hold onto the hope they are what you need, as time gos by and the la<x>yers of reality are pealed away you are left with less hope to keep you warm,