It was never really a love triangle
Jeremiah's Cabo incident isn't even what makes him the wrong boy for Belly. It's more the final nail in the coffin.
He has never, not a day in his life, had true, genuine romantic feelings towards Belly and you cannot convince me otherwise. To him, Belly is the one thing he can win over his brother. The one thing he can have that Conrad doesn't, the one person who should choose him over Connie. Let's be real, Laurel is Conrad's person, Steven is clearly closer to Connie and likes him more, their dad (who Jeremiah clearly idolises in a way and refuses to see in a negative light- despite him being a terrible father and husband) cares more about Conrad, Belly has been in love with Conrad all this time. Even Susannah, who yes, absolutely loved Jeremiah, clearly held Conrad to a higher regard.
So here's this final person in their family who he can take advantage of, because she's been in love with Conrad, but he hasn't been very direct with his affection towards her (because he has been affectionate with her, just not direct enough to clear through her insecurities and make her realise it). So what does he do? Goes on a "Conrad sucks" mission, trying to sabotage him in front of the person who has always seen this ideal version of him. Everyone but Belly understands that Conrad is human and isn't perfect. So she's the one that will struggle to cope with having the rose tinted lenses snatched from her face so that she can see the real him; broken, struggling, often cold- even towards her.
Mr. Jeremy Fisher swoops in and takes every chance he can get to convince Belly that because his brother doesn't go around acting like a puppy the way he does, he has to be the world's biggest jerk. Which is funny considering that title so clearly belongs to their father, who he always makes excuses for. Somehow that well of excuses dries up when it comes to his brother.
This very obvious but mostly underlying issue he has with Conrad, rooted in insecurity and jealousy, permeates into his feelings for Belly. He's not going after her because he truly wants her, it's also because he wants to hurt Conrad. Wants to win over him, for once in their lives.
He sees them almost kiss on the fourth, and knows that he is about to lose the one person in their family (I know it's weird to call them family considering ykπbut clearly they're a family in a sense) that could still love him more. So he sabotages what would have been their first kiss (sabotage really seems to be his brand). So at this point, he knows Conrad most likely reciprocates Belly's feelings. The normal thing to do in this case would be to let your brother, and your best friend who's been in love with him for years, to explore their feelings.
But clearly, Jeremiah doesn't really understand normalcy when it comes to Conrad. No dear, he is far too blinded by the envy that can only exist in the heart of a younger sibling who knows they will never be good enough. And so he pursues a "fling" with Belly.
Two other point that clearly demonstrates the difference between Conrad and Jeremiah's feelings:
- Conrad is jealous regardless of who Belly is interested in. He's jealous of Jeremiah AND Cam Cameron. Because he actually likes Isabel, and will thus see everyone as competition. He's not just jealous of Jeremiah because what he feels for Belly isn't rooted in some lame siblings rivalry. Jeremiah on the other hand did not care about Belly being with Cam, because at the end of the day, to him, it's not about Belly being with him. It's about her not being with Conrad. It's about making him lose something. Lose her.
- Their reactions when Belly chooses the other brother. Belly couldn't even enjoy her relationship with Conrad, something she dreamt of for years, because Jeremiah was acting all broody over a few make-out sessions. He didn't care that he was hurting her by acting like that. Conrad on the other hand, does his best to put his hurt aside at the end of the day (and don't bring up the car ride to the motel because he actually should've reacted worse after finding them doing all of that on his car, on his campus, while she was in his sweater). Conrad wants Belly to be happy, because when you love someone, you do your best not to be selfish. Not to stand in the way of what they need or want. Something Jeremiah can't relate to, because he doesn't really love Belly. Not in the way he claims to.