The requirement for this is simple:
Be at a 2-note level friendship with one of the eligible bachelor/ettes.
View their second-note cutscene.
Once the cutscene concludes, you shall obtain the trophy. It requires more than simply having someone at two hearts, as Dean remained oblivious to my advances before I saw his second heart event in the Mountains.
A quick way to earn Friendship Points (FP) with someone is to use your hoe on the beach when you unlock it and give them the unearthed Seashell. Everyone seems to love the gift, meaning it's just a matter of burying them to their necks in these delightful beauties.