What makes people return to a relationship which is unhealthy?

If it's based on lies, insults, someone being drunk constantly feeling alone regardless of whether if it's true.... why go back?? I don't understand? Maybe, some may feel I'm speaking from a place of priveledge or lack of knowledge... if you say love is the reason is it really love? Or is it want, the desire for it to be love for there to be a reason, and/or it just familairity? Knowing that and not wanting to change a stupid routine where you're unhappy? What is it? Please reason with me... because I truly don't understand!