Why did he leave me? After 2years?

I dated my boyfriend for 2 years and we were very much in love. Now granted we fought. Alot. More than most couples, but we still loved eachother enormously, and a couple weeks ago, we got into another argument, and he left me. This time for good. I cant just throw away 2 years like nothing, but he seems perfectly fine and going on with his life very nonchalant. I just want to know, what could make someone leave just like that, after two years? All the memories, and the things we went through seems like it meant nothing to him.