I'm not sure if that reaction is because Rusty still has feelings for Jill, I think it's more that the fact the MC has conquered the apparently unapproachable Jill is a reflection of his own inability to do the same, get love and have a stable relationship. He is a Burgmeister, he was born to do that, he was in the best place to do that... but he couldn't. And after what happened with Micha, he probably sees himself as a failure. And the fact that the MC is so much better at everything, leadership, management, etc., doesn't help.