There's nothing free in this life
Salvation, salvation, salvation is free
They met each other at the right time. After the loss of her mothers (notice the plural) and a psycho stalker Peyton needed someone to save her, and Rachel, with her fear of commitment and her self-confidence issues, needed someone to save.
Their beginning wasn't easy; it had taken a lot of fights and frustrations to accept their attraction towards each other, and when they finally gave in they had to deal with something more scaring, the rest of people, but especially, Brooke. If she found about their relationship it could mean the definitive loss of her best friend to Peyton, and the loss of the only real friend Rachel had ever had.
But, soon Peyton realized that it might be worthy. After all, salvation is not as free as that song says.