For me, it was because I was so fueled by my desire for a "real relationship" that I forced myself into this one while ignoring every red flag there was.
I made the choice to move to be near him, I made the choice to wait for him, I made the choice to sacrifice every emotional and physical need I had until he was ready.
Turned out he was never going to be ready.
I saw him exactly as I wanted to see him, not who he really was. He only did to me what I allowed him to. I am certainly NOT saying that some of his choices weren't manipulative and self-centered, but I should have walked away long before I did.
There are many reasons in my life's history that helped to form who I am today, But for me in this particular relationship, that is how it got so bad.