Most Helpful Guy
It might be guilt or regret, but the truth is that usually the guy just wants to keep the woman as a possible option and is trying to keep on fairly good terms with her so that he can make a move when the better options run dry.
I admit, I've done this many times before and it's not a particularly nice thing to do. My recommendation is that if this is happening to you, feel free to talk to the guy if you want to, but be wary of anything he says, especially if it's some cheesy crap about how messing things up with you is the biggest regret of his life.
Most Helpful Girl
In my experience, guys usually reach out when they want some free sex on the side.
I've also never had a guy come back who wanted something for real again. Mostly they just want someone to fill the void and want the benefits without commitment again.
There are always exceptions though but there's no sane or logical reason to believe a ex will come back if he dumped you. That doesn't happen as much.