Ultimately, it's a form of fanservice aimed at the fact that virtually every person in the show's target audience has fantasized, at some point in their lives, to the effect of "Wouldn't it be great if I had a significant other who'd completely do my bidding and love me no matter what?"
The best way to determine if this is the case is to apply the Des Anges First Law of Writing. If you asked yourself "What would this character's personality be like if they'd never met the protagonist?" Or, "What do they talk about and do together other than wax poetic about how much they're in love?" and there's no real answer, they're a Satellite Love Interest.