To Marcelo, suffering and death are just facts of life, and he doesn't see why they're such a big deal. He's not a jerk; his brain just works way differently from most people's. But when he learns about Ixtel, he learns the true meaning of suffering: this is a girl who can't do basic mammal stuff—like eating, for example—without pain. By putting himself in Ixtel's shoes, he realizes he has to do something; this girl needs a Superman big time. Even though helping her may very well hurt his family, he realizes he doesn't have a choice. What else can you do when you're Marcelo in the Real World besides put on your cape and swoop in?
Marcelo may not understand human interaction, but in empathizing with others' pain for the first time, he learns about human feelings, which is a step in the right direction.
Arturo compounds Marcelo's suffering by forcing him to work at the law firm instead of listening to what Marcelo really wants. The fact that Marcelo doesn't realize his dissatisfaction as suffering doesn't mean it's any less profound, even if it's not as profound as Ixtel's.