Why did he fall out of love with me?

18 months and we were getting pretty serious. He always said we were perfect for each other. We didn't fight that much, everything seemed great.

I was gone for 2 weeks, and when I came back he was distant and irritable around me. He says he fell out of love in a week's time, and shortly after developed a crush on another girl.

I know he did not leave me for her, she wouldn't even date him. Gave him the whole just friends thing. But even still, he said he loved her after a couple months!

Months later, he started staying the night at another girl's place. (and this girl is like the opposite of me in a bad way) That only lasted a week or two.

He's alone, has less emotional and financial support, and no real steady family to go to like he had with mine. No reason for him to leave other than he really did just fall out of love in a week (after 18 months together!). He doesn't even talk to me.

What happened? How could he just fall out of love like that?

He says he doesn't know what he wants, couldn't handle a relationship, and does not love me, but isn't saying it could never work out.