I was just skimming through the book “Watchmen and philosophy” (Mark D. White, William Irwin) where Christopher Robichaud touches in on the subject in the title of this short essay.
If you are not familiar with the character, and thus do not quite get the subject, I suggest you go read the graphic novel.
It is obvious that Dr. Manhattan has problems with his emotional attachment to humans, and there of course can be multiple reasons for this. As he is a fictional character, we must also take into account that Alan Moore - a great artist he may be - may not have foreseen absolutely all possible aspects of extrapolating the characteristics of his characters into the philosophical realm.
There is however one possible explanation, which for me (since reading Watchmen for the first time in 1987) always has stood as the main contributing factor to the Doctors emotional detachment, which I feel Robichaud brushes off a bit to easily.
“we might think his moral attitude towards humans is simply the result of his supreme intelligence and power, without having anything specific to do with his emotions” he writes, “he can read atoms after all”, but he disregards this theory because “the world he’s in contact with is not a mysterious one that ordinary physicists aren’t aware of. Stephen Hawking is well versed in the scope of the cosmos and our tiny place in it, yet he doesn’t consider human beings to be morally on a par with rubble.”
Here I believe he underestimates one aspect, and makes one error in his analysis of physics. One thing is that very many well known physicists actually have been not very nice people on a personal level. I have not made any statistical analysis, but we must not disregard the fact that this may be more than a coincidence. (Had I not been a physicist myself, I probably would not have dared to state this claim.)
On a slightly more serious note, I believe Robichaud underestimates the huge psychological difference that exists between knowing something, and living in something. Most physicists know the vastness of the universe in both space and time, know of the elementary particles we are composed of, and the forces that govern them, know the Heisenberg uncertainty principle, and general relativity and so on - but they still live in a macroscopic world that for the most part seems continuous, and in a time that seems linear, just like all other humans. And they for the most part in their daily lives are guided by the same human instincts and emotions as other humans.
This does not apply to Dr. Manhattan. The physical reality is thousand folds more real to him, than to the most ardent human physicist, and we should thus expect a completely different attitude towards both life and other people from Dr. Manhattan.
Now to the slight physics-error: Dr. Manhattans universe is actually not quite like ours. As we can clearly see in Volume 3 of Watchmen he moves seamlessly through space and time, and the past present and future does naturally not hold the same meaning to him as to (other?) people. He lives in a universe where past, present and future are fixed, and he knows what it all is (except when Ozymandias is bombarding him with tachyons - nice little trick from Moore to avoid a few of the ordinary physical paradoxes when time is involved). When you already know what is going to happen (or rather you’re doing it all “at once” - the human language is so closely linked to a linear understanding of time that it is hard to write about these things) the emotional approach to what is happening naturally might suffer. As I see it this however probably brakes with both Heisenbergs uncertainty principle, and entropy, and probably a few other things as well, but don’t let that ruin a good story.
(As I know many physics-nerds also read comics, I would appreciate insightful comments. I don’t know whether seeing the film will be sufficient to understand the questions at hand, but hey - just give it a go.)

