Angeal just recently passed away and Zack has been mourning his absence. He was distraught, his grief was quite obvious to everyone since he's lost most of his cheerfulness.

You went to check on him under Sephiroth's orders, though you're quite contrary to such idea since you wanted Zack to have his privacy... When you entered his room, he was sitting crisscross on the bed, crying silently and clutching his chest as if he's in deep pain. It took quite a toll on you to see him like that...