it was actually what i first wondered when i first watched the series. i have my own story in mind about what happened between stan and tina; after milo gets the mask back(the ending the The Mask), stan is at first stunned and tries to hide it from tina as they were now a couple. but after seeing all the crime in edge city, and the temptation to release his inner desires, stan always goes off being the mask. but tina doesnt want the mask butting in between her and stan, literally. she confronts stan and tells him that its either her or the mask. stan says he cant just leave the city to the crooks, so tina decides to leave him and pursue her career as a singer, while stan is left as being the crazy vigilante.