If Mark wants Ethan back, he'll have to start showing a sexier and freakier side of himself.
I cannot say for certain why they parted ways, but based on what I've what I've seen from Ethan and what I've seen from Mark, Ethan is a Rachel (wild, fun and free) and Mark is a Ross (clean, polished and dry). And just like Ross and Rachel, Ethan and Mark have danced the on-again-off-again tango more than once, but unlike the show where the couple presumably live happily ever after, at this point, disco has a better chance of making a comeback than the love between Mark and Ethan.
It's very obvious that Ethan doesn't want a schoolboy who sings in the church choir, but a bad boy who pulls his hair while he rails him from behind. The way I see it, Mark has a choice: Move on with his life without Ethan being a part of it, or change his core character to bait Ethan back.