Seth had spent a good portion of last night listening to Zack alternate between talking quietly, crying and yelling. His friend was confused and hurt and, yes, happy all at the same time and eventually Seth had managed to calm him down and had curled up on the bed in the spare room with him until Zack had finally fallen asleep.
So really it shouldn't have been much of a surprise that at some point Seth would walk through Xander's door.