First I think this may qualify as a whole reading if only two cards were pulled.
Second if I am to understand your question: "why the relationship between R and P is different from the other relationships, R has experienced"
Is this meant to explain R and only R or the relationship? I would take this question to be about R and what it is about the relationship R is involved in that makes it different. Is that right?
I would suggest then that R is perhaps a fire sign as the Emperor is Aries. He is normally quite closed off from both his feelings and others. He may have a bit of a barrier between himself and others. Others may see him as mature, controlled and in charge. He may even have a position of power such as managerial.
The reason this relationship seems to have blown his socks off is because this other person has brought out feelings that he had previously ignored. He may become much more in touch with himself and his feelings and the way he appears to others. He may also have new interests brought upon by meeting this other person. I get the feeling that he is drinking this other person in as these are such opposite cards so there may be a marked internal change within him and a change in his perspective.
This person has perhaps made him reconsider things he took for granted or even perhaps the way he views his life.