When a man would rather quit than put work in to the relationship

Suzette
Well my fiancé or ex fiancé called it quits. How do some people just quit so easily? Why is it so much easier to let go of someone who has moved mountains for you than put in the work for fix the issues to make the relationship stronger? I was with this man for two years. Through out his health issues which were great. He has a serious disease and I was there for ER visits, hospitalizations, surgeries.  I have driven 50+ miles 2-3x a week for two years because he was unable to drive to see me. Never asked him to come see me. Through his insecurities and so forth. Never asked, complained or demanded anything of him. And he calls it quits because I brought up an issue in our relationship. I'm trying my best to put my tough face on. But my heart is broken.