Everyday by David Levithan. Every morning A wakes in a different person's body, in a different person's life, learning over the years to never get too attached, until he wakes up in the body of Justin and falls in love with Justin's girlfriend, Rhiannon.
"It's bullshit to think of friendship and romance as being different. They're just variations of the same love. Variations of the same desire to be close." -Naomi and Ely’s No Kiss List, Rachel Cohn and David Levithan