This is actually really sad for the guy. It's in his job description to remove as much stress from her life as possible: that means helping her deal with her physical, mental, AND emotional life. The fact that he must also cater to her emotions distinguishes him from a butler. Now, if he has to be around her so often, then he must act like someone who doesn't induce stress for her cuz otherwise he'd be defeating the whole point of his job. So in essence, he has to resemble some form of the perfect man for her, and what woman (or person for that matter) wouldn't fall in love with that? Thus, the guy must know that any affection from her is not towards him: it's towards the man who he plays in front of her.
A kept man, huh. Certainly sounds like a fulfilling job. But man, he certainly is a professional. Being good at sex is one thing, but I found his reaction to getting slapped rather respectable. Rather than reacting negatively, he took it as a good thing that she was letting her pent up emotions out. That ending though. "The kept man part is the one who has her affection", huh. That almost sounds a little depressing, like he is saying he is nothing more than his job title.
I think what he means is that the man he portrays to her is not who he really is. So if she did fall in love with him, she wouldn't truly love him for who he is. Think of him similary to a butler; it's his job to tend to her needs, no matter how he is as a person. So if she fell in love with him because of what he does as a Butler, she's really in love with him the butler, not him the person. It's tragic really.
In reality though, they missed a hyphen on that last page. It should have read: "The kept man-part is the one who has her affection." These editors need to pay attention to the details! LOL.
"The story you have just read or are about to read is true ... the names have been changed to protect the innocent."
Seriously though, it's not inconceivable that having been pressed into taking over her father's duties likewise forced Erina-san to get all tough to handle it all; hence her stress. Of course, it's also quite possible that her mother had another *cough-cough* agenda for engaging the services of a "kept man" (the "I Want Grandkids" set-up).
Credit goes to male protag-kun for not instinctively lashing out when he got slapped ... that probably required much training and discipline. And in the end, it seemed to work - whether Mom had another agenda in mind, we may never know ...