yesterday we were just children playing soliders

What if, when Harry heard the Slytherin portion of the Hat's song, he payed attention when the Hat sung perhaps in Slytherin, you'll make your real friends? Imagine this: a little boy- beaten, and ignored, for his entire life. A little boy- friendless, and lonely. A little boy- desperate for the kind of friendship he had read about in books, seen on the television screen.
When the Hat offered him Slytherin, imagine that Harry said yes.