I constantly wonder what Aro was like as a human to have devoloped his gift. Like??? he's like an actual vampire Sherlock Holmes, but psychic, I guess??
I think your Sherlock Holmes comparison is very appropriate. Human!Aro must have been incredibly observant and also something of an amateur psychologist. He spent a lot of time studying emotions and motivations and– quickly realizing how useful that was– worked to develop that skill further.
Moreover, I’m pretty convinced that Aro was a second son when he was human and markedly inferior in comparison to his older brother. Usually, the trope goes that the second son is just as good– if not better– than the firstborn, but tradition subjugates and marginalizes him. Aro didn’t even have that cold comfort. His brother was just as bright, but more charismatic, better in battle, and as a result, more politically influential than Aro could dream of being. In order to compete, Aro began really noticing what people wanted, needed, and thought, with an eye for exploiting those rather private musings.
Finally, I think Aro’s quest for immortality really honed his perception. Nowadays, we know that nothing can make you physically immortal (or if it can, science hasn’t got there yet). During Aro’s human life, there were mystery cults and weird medical practices and rumours everywhere, with no good way of fact-checking any of them. Actually finding real, undying vampires made Aro an expert at judging intentions and motivations.