The way adults project their fantasies onto children.
She thought - she seriously thought - that her best friend's son was my best friend. She believed this for years.
The truth was that I tolerated him, because she wanted me to be his friend, and because children are practical enough to play with whoever happens to be at hand, especially when they have no choice. Once I was free to choose my own companions, he became someone to say hello to in the hallways, nothing more.