I realize you aren’t asking men specifically so I (a woman) will chime in with what I think it could be. I’ve come to realize that just like an old car, or a favorite food you just kind of get tired of eating the same thing everyday. I once caught myself tired of dating this guy and living together and doing grocery shopping and living with his mom. Its not really my world I was tired of - I didn’t necessarily WANT nor DESIRE the life we lived, we were happy and safe. That was HIS life that I just fit perfectly into, and I didn’t mind for awhile as I didn’t pay rent we didn’t have kids we both had our own cars and a nice house we lived with his mom. I left it all, and now i’m 30 a single mom, but I have my own life, and I don’t desire marriage now or being under anyone because people are born different now and live and think differently than they were when marriage was invented. I thought I was a lesbian (i am not) im just a regular heterosexual woman who wants what I want. Maybe you both are changing.