Star and Marco have been friends ever since she moved to Earth from Mewni. She never expected a whole bucket load of trouble to cause her to develop feelings for Marco.
When stuck on Earth, Marco couldn't find his place. He thought about returning to Mewni, to Star, but he didn't have a place there. Deciding he had had enough, he left to find his own place with no intentions of returning.
But, when Star opens portals in his meteor shower, he finds himself back in the world he ran away from.