Ex is dating someone else
In the hurry to do this, he probably didn't bury those emotions very deep.
This is where you'll come out on top, and he's going to glorify the days where you both had happy times together.
As your ex's new relationship crumbles away, he'll be looking for someone to talk to.
He'll also want someone he knows and is familiar with, and that person will undoubtedly be you.
You'll be the comfortable, familiar choice when it comes to whatever his needs may be.
Additionally, there's a good chance your ex was thinking a lot about you during the time he was dating his new girlfriend...