Every reason is valid if it makes the person's personal reality hell.
I spent 11 years with this person. We built a family. I stayed by his side while he built his career. When my mental health didn't allow for his sex life anymore he cheated, left me for her, and took away the future of security of comfort and safety we had built. This was my best friend... it was someone I loved. It destroyed my self worth, and left a whole that physically hurt it made me so sad. It shattered reality beyond what I could cope with.... love was supposed to be forever, and now I watched him give what we had to a new woman like I meant nothing, like it was trash. I don't even feel like I'm worth being loved anymore.
I hope you are strong enough to not feel the crushing pain of a broken heart, but not everyone is.