Until the end I still didn't understand Quaid's attraction to Zack. He's an arsehole who finally comes to his senses because he's afraid of being left alone. It's not even about love.
"If it were over I'd smile back, Zack thought. I'd ask him his name, where he was from, how long was he in Sin City, would he like a drink…
He took one more regretful look at the boy and left the gardens. See? he thought. The things I give up for him. He has no idea."
What a jerk.
I still think Quaid's better off without him.