At the end of Season 2, Mon-El left Earth after lead was released into the air to stop his Daxamite parents from taking over the planet.
The lead was toxic to all Daxamites and so he had to leave to save his own life.
While Kara and Mon-El had a fairly tumultuous relationship, it’s admittedly been the most the Girl of Steel has gotten in terms of romance on the show.
Kara was only with Mon-El for a short period of time, though.