I'm glad that it does come off as logical. Jayne is a blunt, coarse man. I love him, but I can't call him a thinker or a cuddly or anything like that, and for him to be able to trust his partner, he needs to know that his partner can be just as blunt. River sees that. Mal could have provided that bluntness, but like I already said, I saw Jayne putting out submission signals in canon, and Mal never picked up on them, so time for someone to step into the gap.