When I think of sex, when I fantasize about sex, my brain tends to choose a perspective or two. One is my own perspective. I visualize things as if I'm seeing it myself. However this is not my brain's preferred perspective. My brain's preferred perspective is from the guy's point of view. To the point of imagining what he would feel.
My brain will switch back and forth between the two depending on what is more appealing.
From the man's perspective I can imagine what it would feel like to thrust a cock deep down a throat. I can imagine holding a woman's ass as she's being fucked from behind. To be honest the woman is always me. Yes, this seems weird even to me. I can picture my (man) self fucking my (woman) self. But my 'man self' is not actually me. It's just me seeing through my partner's perspective.
I imagine that on reason for my empathy; this capacity to feel what he is feeling, comes from porn. The imagery we see is almost always from his perspective. It has been engrained into my fantasies. I have never seen a porn video eroticize men like most porn does with women. I would be curious to see that. The only types of videos that do are gay porn, which is probably why I don't mind (though I don't actively search for it) gay porn.
Another reason comes from my innate need to please. I want to please my partner so much that I embody his person's perspective. How else will I know if it pleases him?