My friend and yours, the Discerning Dom, has written on the threesome from the male perspective, including why it's desirable to share your submissive with another man. I understand all of those reasons. I see them expressed by my Owner - He wants to feel that he can use me and own me so completely that I'll fuck another man for him, He wants me to express my inner slut for him, etc.
But why do I want to share Him with another woman? I feel it very strongly, but I cannot explain why. Remember the experience that we had this spring with that other couple? I didn't write about one part of that experience - the part where I sucked my Owner's cock with her. I was just about to start sucking His cock while the couple watched when I was overcome with the strongest feeling - I wanted to see her do it too. I wanted Him to feel that. I wanted to see His cock in her mouth. I wanted to share all of that with her and embrace them both in that moment.
I got my wish. I was so pleased to look up at one point and see my Owner reclining on the bed with his hands behind his head and this look of utter bliss on his face. He looked like the happiest man on earth.
And I have felt that desire many times since. We are about to embark on something new, possibly with someone we both can share. I want so many things with her and with Him, things that I want to see her enjoy with Him and ways that He can please me by pleasuring her. I cannot tease out all of the threads and understand why. I am happy that I feel this way because I think that it means that I'm growing into my relationship and comfortable with all of the ways that my Owner and I can explore our love for each other. (But there is a tiny part of me that thinks that my desires are fucked up and maybe wrong somehow.)
I know that part of it is simple - that I want to share Him just so I can experience his pleasure second-hand. That is part of it, surely, because I am nothing if not a people-pleaser. But I am usually also a jealous person and I would have expected that I would want to please Him myself, to know that I provide Him with the most pleasure in this world, not anyone else. But when push comes to shove, I'm not jealous. I'm generous and I want to share Him.