THIS
I'd also add that they feel Belly owes him constant reassurance and excuse all of his poor communication and tests, putting the blame on her for accepting his proposal and all that, because he's insecure about how she had a crush on and dated Conrad. His insecurity cannot be wholly Belly's responsibility, after four years **that is on him**.
To still want Belly and Jeremiah together because his insecurity is "justified" because she does have those feelings for Conrad, even as she was passing all of his tests and none of it was ever enough to satisfy him. It's just this very inequal dynamic, and so damaging to Belly's psyche. Because it kind of reveals that Jeremiah operates from seeing the ways she disappoints him, and needing her to prove otherwise because she's his sole basis for self esteem. It doesn't matter how much reassurance she gives, he'll always fixate on the ways she's not living up to the impossible standard she has to meet for him. He *knew* she'd always love Conrad. If you don't believe in deliberate action over inherent feeling, then it's grossly unfair to date her in the first place.
And the thought that it's cruel for Jenny to create this character who's inferior at almost everything, who isn't favored by anyone, just to have him "lose" in the end is laughable. For one, he's not the main character. And additionally, that's the point. The theme is that you cannot force what you feel. Love cannot be a form of justice. Other people cannot be relied on to fill a hole that exists in yourself. Despite Jeremiah being less accomplished on paper, he IS loved. But because he fixates so much on the comparison, well beyond the way other people compare them (which is really just Adam), he cannot see that.
It's all about self love and compassion. Belly has to love herself to choose what she wants. Conrad has to love himself to be willing to let others love him and to be vulnerable. Jeremiah has to love himself to realize and accept all the love he's had all along rather than fixating on what he feels he's lacking and lashing out at people for not conforming to that.