Once you hit six hearts and especially by eight hearts, those characters have lost interest in their former possible pairings and are interested in the farmer. Even if you don't date them.
Yes, the result is a town where every bachelor/bachelorette is interested in exactly one person and nobody else will settle down. Some are more subtle than others about it but it's there. Especially if you give them a bouquet and date them. And they can act jealous if you gift other folks.
I've said elsewhere this is a major thing that got me into adding mods; they allow for intentionally staying platonic or dating multiples honestly. Or both. (and ones where they do date each other snd even marry).