No reason at all

It stings.
Sharp, tangy, bitter water laced with chlorine is filling up his ears and his mouth; now his nose and his eyes. He can't feel his legs or his arms, can't move them, can't keep his head up, but his eyes and nose still sting. He can move his mouth, can still scream, but what's the use? His ears pick up the nothing that means he is alone.

Nearly alone.

He looks up. All he can see through the murk of the water in the dim light fractured on the walls from the lights below is a single figure, and he knows who that is. The lone figure on the edge of the pool is not here for him. He isn't going to help him. Why would he? He'll grab his phone, get his friends, save them. He will leave, and the one in the water will be finished.

The water is cool on his face, over his face, above his face. It's dark, quiet, still.

Until it's not.

There's a crash, a movement, a splashing sound, and he feels arms around his neck tugging him up, yanking him from the stillness into the noisy air. The man from the ledge supports his weight, treads water, lets him breathe.

He had no reason to help the one in the water.
But he did.