Pluto represents transformation. Sun can represent life direction and the conscious self. When there’s a square, opposition or conjunction between Pluto and Sun in a synastry comparison, it seems like Pluto-person is trying to transform the Sun-person in terms of his/her entire being, life direction and life purpose as well.
The Sun represents the hero’s journey, quest, say destination A. So, when Pluto aspects Sun, it’s almost as if Pluto person is saying to Sun person “No, don’t do this. Don’t go to destination A. Go to destination B instead.” Pluto also represents power and sometimes power struggle. So, Pluto-person could unconsciously or consciously weld control over Sun person. It’s almost as if the path chosen by the Sun-person never quite satisfy the Pluto person. Indeed, Pluto symbolises depth. Think of a bottomless pit. It never gets filled. It can be rather frustrating to the Sun-person as he/she has to keep transforming his/her life based on the dictates of the Pluto-person.
Imagine if it’s the guy who’s the Sun-person and the woman who’s the Pluto-person. It is possible that the man feels that he is under the control of the woman. The dynamics on the relationship would depend on the cultural background, the era, other aspects of the synastry chart, etc.
Based on the above, it seems that there’s no positive in a relationship with a Pluto-Sun hard aspect. I think it’s equally likely that the Sun-person illuminates the deep emotional recesses of the Pluto person. It’s like shining a torch into a pit. The Pluto person might be enlightened as a result of the relationship.
Also, I can’t help but think that Pluto darkens something while the Sun brightens something. The reality is that, at least in the world of colours, when you introduce black into anything, the lighter colour inevitably gets darkened. Or grasps, “tainted”. Perhaps that’s how I feel of a Pluto-Sun relationship. Darkened instead of brightened from the perspective of the Sun person. But who’s to say that the world of dark colours is “bad”. It’s a matter of preference. For example, my partner loves the dark and dark colours. As for me, I gravitate towards the light.