There’s a lot of variables to this, but often it comes down to the fear of being alone. You believe that even though this love isn’t healthy, it’s the only one you’ve got so you might as well stick with it. You may believe this will be the only love you’ll ever have. The irony is that when you develop self respect & standards for yourself in relationships, you’ll attract even more people into your life.
I also want to add that it depends on how you interpret love and relationships. For me, it takes more than just my feelings towards them to influence my decision to be with them. There could be all the butterflies in the world, but if they don’t have the same long-term values as me, I will not pursue them. I’m a child of divorce and I am extremely picky about who I choose to invest my time to; because I’ve seen what can happen if I don’t.