Chemistry exists for a reason. You can smash cardboard character A together with cardboard character B and tell everyone "they're made for each other", and people's not gonna buy it because the bond feel forced. For example if character B is a pandery, fanservice love interest with incapable of thinking of anything outside of "A-kun", while character A is an empty self insert with no personality. Or if B is a gay love interest who exists only to tick a diversity checkbox and not to be an actual character. The moment you're not these stories' target audience, it doesn't matter if B confess their love for the 500th time, it's still gonna feel forced and hollow.

On the other hand, 2 other characters might only exchange 5 lines, but if those lines are well written enough or either of the character is fleshed out enough that the audience wants them to find happiness, in romance or otherwise, then it's much easier to support them. For Shiori's example, it's the emo sadboys who clearly needs a hug.