Actually, Superman used to show quite a bit of human empathy in the Golden Age (once he was past, eh, his early rambunctious phase where he tossed around cars to demonstrate traffic safety - and even then, he had his moments). Jerry Seigel may have written him as a tad remote, but his Superman was a really nice guy - as were his Lois, Jimmy, etc. The problem is, after that came the Weisinger area, and Weisinger (this is true) actively HATED the character he was in charge of - later in life, he went through therapy, and it turns out he had some series inadequacy issues which dealing with the adventures of the strongest man in the world only exacerbated. Combine that with the fact that he himself was a thoroughly obnoxious character, and it's no wonder that Superman was basically portrayed as a jerk for so many years - Weisinger projected his own dickishness onto the character, and later writers took this as their cue to do the same thing. Morrison didn't ADD the human heart; it was there all the time - it just got obscured by the Silver Age.