How can a relationship end so fast and how can they move on so fast?

I start not understanding them...the men i mean.
How can they move on so fast after a relationship? How can they switch girls and be just as happy about it as they were with the same girl for a year?

We just broke up and i keep crying at nights, i don't eat, i don't sleep...that is all i do. And all i can think of is that he's not next to me anymore...

And he is soo cold out of a sudden, he talks to me like i was a stranger and i feel like he completely rejects me. It hurts like heck.

All i can ask is why is this so easy for them?