"It understood you just so far as you wanted to be understood, believed in you as you would like to believe in yourself, ans assured you that it had precisely the impression of you that, at your best, you hoped to convey" - Nick goes on a tangent whilst admiring Gatsby's smile, it's clear his affection runs deep for Gatsby which is a stape notion even after Gatsby dies. Nick is trying to fit in the West Egg and Gatsby is making him feel welcome.