I had a friend who walked away because she wasn't emotionally, mentally, or financially prepared to care for the child. She believed that she had to be all or nothing. When she "failed" at being full time CP, she handed the child over to the dad and walked away. After MUCH coaxing from the dad, he convinced her that she wasn't a failure for not being ready to be a CP, that their daughter needed her and he wouldn't judge he. (Luckily they were long time friends before the pregnancy happened so they had a basis from which to coparent even if marriage wasn't going to work out.) The daughter is now an adult and has an awesome relationship with both parents and both step-parents. It takes a lot of trust that the NCP isn't going to try to take the child away later for a CP to facilitate the relationship. If you can't be married to someone, chances are, there's not going to be a lot of trust.
I facilitated a relationship between my sons and their dads. When my 16 yr old OS wanted to "never go back there again" I encouraged him to talk honestly with his dad about wanting to do other things (like set up for VBS and hang out with friends or go to RM to visit family) rather than lie to him about "working". 4 months later, my son
moved out of my house and in with his dad, being talked into, fully encouraged and convinced of his "righteous rebellion". Took him 4 years to realize that he had been manipulated and used. We have a great relationship now. I had often wondered if I knew my ex and his wife were going to succeed in their mission to turn him against me, would I have helped facilitate that relationship all those years and especially that summer when it was faltering. I'd like to think I would have but honestly, it would have been tempting to avoid 4 years of heartache.