I think book Sophie's inner monologue regarding Howell could be summarized as "fuck, I love that guy."

And it's just yeah.

I mean, Sophie just learned how to enjoy freedom (from being old). To just be her weird self, doing whatever she pleases and not hiding a bit.

And in her freedom, she realizes she's in love. The freedom being free of all the emotional webs, the burden of thinking about your every step, your role in a society. Old age gave her that. And what she ends up doing anyway? Having feelings! The thing she wanted to escape from! But oh! She's not losing her freedom a one bit by being with him! She just has a guy to share her weird self with, who loves her back.