There could be any number of reasons why he left. It depends upon the man, the circumstances and the dynamic between the couple. We can ask ourselves questions such as: 1) Has the relationship been deteriorating for some time? 2) Am I too clingy? 3) Are either of us too jealous? 4) Am I financially responsible? 5) Am I clean; do I have good hygiene? 6) Do I try to be healthy? 7) Do I try to improve my life in small ways? 8) Am I a good listener? 9) Am I interesting to talk to?
On the other hand, maybe the guy isn't one to stay with anyone for very long. Maybe he is too demanding or has unrealistic expectations about how a woman should treat him or how she should look and dress. Or maybe you were never really the girl for him. Did he leave without an explanation? You might ask him why he left and to be honest "because you want to know why." However, be prepared to have your feelings hurt if you don't like the answer.
Your best bet, though, is to move on with your life. Try to improve yourself in small ways. I'm sure you can do better than someone who leaves without an explanation. Good luck to you.