Not wanting to be alone is probably the #1 reason IMO. I remember when I was seeing an older man (very long term relationship, 10 years) he took me so for granted it was incredible. I just went along with whatever crumbs he threw my way, but I told myself that if someone else came along, I was out of there. He was not going to take the relationship any further and let me know it, and I in turn told him that if another opportunity came about, well...I might go for it. Well, I did meet someone else...and I immediately broke it off with him. Never looked back either...so for me, I think I just would have rather been with someone who was not emotionally there than be alone. I have grown up infinitely since that time...I would rather be alone than put up with that. Cause I not completely unhappy being alone...have lots of great friends in my life.