The Odd Woman and the City by Vivian Gornick

"My friendship with Leonard began with me invoking the laws of love: the ones that involve expectancy. 'We are one,' I decided shortly after we met. 'You are me, I am you, it is our obligation to save each other.' It took years for me to realize this sentiment was off the mark. What we are, in fact, is a pair of solitary travelers slogging through the country of our lives, meeting up from time to time at the outer limit to give each other border reports."