I feel like this is a bit of a hot take, but I really like Jeremiah. Now, do I think he and Belly should be getting married right now. Absolutely not. Do I think they could last even, also no. But I really like Jeremiah as a character.
Fair warning: I've only seen the show. I haven't read the books (yet)
To me, Jeremiah is so very insecure and feels inferior to his brother(this is literally stated in the show by Belly). And if you look at how he grew up, that isn't strange. With his dad brushing him off and praising Conrad.
During Conrads conversation with Susannah we saw this season, we learned that Jere has always been charming and has used that to get his way. It's not that big of a stretch to say that's how he gets validation for himself.
Now for the Belly of it all. Do I think he was only interested in her because Conrad was. No, definitely not. I think it's very clear he loves Belly. I do think a small part of the reason he loves her is she picked him. I think it was, for him, an added bonus that he would be "beating" Conrad if he was together with Belly.
Sadly, I think his relationship with Belly reinforces his inferiority complex because The Summer I Turned Pretty has always been Belly and Conrads love story. And I think that on some level Jeremiah has always known that.
Now, of course, I don't know that for sure, but the way this season has been highlighting all of Jeremiah's worst traits and highlighting all the ways in which Conrad is a good partner to Belly, is a pretty strong indicator.(book people, please don't spoil me) Metatextually, if this story was about Belly and Jeremiah I don't think we would've gotten a third season.
I think Jeremiah ultimately doesn't trust himself enough to make the right choices. Which manifests in a lack of spine. He'll push back a little, but in the end, he tends to cave pretty quickly. Especially with his dad.
This is also why him hooking up with Lacie Barone after he thought he and Belly were broken up makes total sense to me. Because that's how he's always looked for validation. Even him keeping it a secret from Belly, while absolutely the wrong thing to do, is very understandable. If you think other people are better than you, you're gonna try to hide your mistakes.
It's also why he fails to shut down his friends when they say or do dumb shit.
Now, let me be clear. I'm saying that Jeremiah's actions are very understandable. It's not an excuse. I think getting married right now is a horrible idea and he's definitely been lacking as a partner. I just think some people go way too far in their critique.
Blowing shit out of proportion. Like him handing Belly a box and then grabbing one himself. Instead of carrying them both himself.
I've seen people call him immature and a manchild. He's like 22 years old(I don't know his exact age). He isn't supposed to have it all figured out. It's not strange that he has insecurities he needs to deal with to become a more fully realized version of himself.