No real reason? He gets infected with a plague that he thinks is caused by Aeris/The Planet wanting revenge against humanity for how its been treated.
Cloud has legit reasons to be emo in AC. Like, Cloud doesn't feel guilty because Aeris died, he feels guilty because he thinks Aeris is blaming him for her death by giving him the plague.
Problem is that Cloud's line of thinking on AC is kinda stupid and unresonable, considering Aeris and the Planet could have killed them all if they wanted at the end of FF7, like Bughnegaggen said. It feels so forced just because they wanted an emo Cloud, same as how Sephirot appearing in AC was also forced as fuck.
Anyways, staying on topic, I would say Cloud and Kain.