Normally I hate most cases of "main boy and girl must be paired together" but Starco is an exception due to how well it's pulled off.
The thing about romance and writing it is that most writers don't understand how to play with both the viewer's psychology and the cast's dynamic well enough. Nefcy may or may not accidentally have struck gold writing Star and Marco. The two, romantic implications or not, already share a powerful and effective dynamic. You can see the two have subtle effects on eachother as the show progresses. Star learns more about Earth culture and learns to be more loving exactly because Marco is who he is. Star brings excitement to Marco's life and proves that he can be that dangerous person he knows he can be so long as the scenario is unrealistically coincidental (which they often are.) The two build a necessity for eachother this way without directly having to say they need eachother. The audience knows, and from there you have a formula that works.
But supposing even that's not enough, they're not always sunshine and rainbows together. They even bring out some lesser parts of eachother at times, and know how to deal with them right. They also know to keep their distance as a pairing for the time being (provided their current, respective crushes) giving the relationship a good sense of progress. Not to mention their characteristics just bounce off eachother super well when talking. You can't deny their exchanges are pretty funny/adorable.
They also avoid a lot of the terrible trope executions (like Star's suspicion of the exchange student being a legit suspicion and not some childish jealousy, and Star encouraging Marco to go for Jackie.) All in all it's a very strong playout of what would otherwise be an uninspired "necessity" for the sake of hamfisted romance. It's fun to watch them simply do things as they develop in their teen years, and they get more mileage than when they talk with anyone else.