When you've got a narrator who says, "Suffering and death do not affect me the way they seem to affect others," (4.34) you know you're either dealing with a psychopath or someone with emotional processing issues. Luckily, in Marcelo's case, it's the latter.
This is a kid with Asperger's Syndrome who's spent years learning how to make small talk and read human emotions in body language and facial cues. He understands what others are saying largely through context. For example, he doesn't get it that someone's mad unless he goes through his checklist of stuff mad people do: "Even when she is angry, like at Juliet for example, you can tell that the anger does not affect her. The reason I can tell is that her breathing never alters. A person who is truly angry has physical reactions that last for a while, even after the event that caused the anger is gone" (8.27).
Those of us who don't have Marcelo's emotional processing issues get it pretty quickly when someone else is mad, and it's often because they make us feel mad, too. Or when they do something scary, we feel uncomfortable; we generally don't do those things ourselves because we don't want to scare people. That's because we've all got natural empathy that's easy to access.
But Marcelo? He doesn't quite grok the nuances. When Arturo tells him that quoting scripture will scare people, Marcelo pulls out a notebook and writes, "Do not pray so that others see M. pray. Do not quote scripture. Note: listen for religious phrases that have become figures of speech. Those are allowed even if not accurate" (5.56). If that's not objective, we don't know what is.