"No, you're not." If Cloud is anything, it's stubborn. He was stubborn about dying himself not that long ago, almost to the point of losing everything. Now he's stubborn about this. It's Zack. The world still needs Zack and not in the form of random floating bits of energy. Maybe it's selfish but damn it - isn't he allowed by this point to be selfish?

"You're still here. You can still fight. We can still fight this. I can have a future and still have you too. It's not an either-or this time."