Because when they are with each other they don't have to care about anything else. The world doesn't touch them; it's not their oyster, nor is it watching them constantly. The boy doesn't have to care about the fact that he's holding onto his girlfriend's best friend and the girl doesn't have to worry about the fact that she's not even supposed to like him. She can just relax as her body melds into his and ignore the feelings that come when his eyes stare into hers.
They sit on the phone with each other for hours some nights, finding the pieces that make up the other person but never venturing into finding out what their relationship is made of. They do what's expected of them in public, their verbal sparring matches almost timed to the second; never deep enough to cause a scar but always insulting enough to break the skin.
No one else knows; no one would ever even believe it. It wouldn't make any sense to an outsider, that guy and that girl aren't supposed to talk to each other without evident scorn let alone lean on one another. It's okay though, because the two people don't even understand what they have. They don't understand why it happened or how it stared but it's the only thing that seems to make sense in the moment.
A comfort that's not technically forbidden but still unwelcome in the petty lives of their friends. It didn't matter when she cried as the rain fell and he grasped her hand or when their phone bills began to creep up, what everyone thought didn't matter when they were alone…even when she kissed him. It only mattered after, when she stopped talking to him altogether, writing letter after letter to explain but never send.
The guilt rose within her just as their phone bills once did and she was worried about it. The feeling wasn't a welcome one; the guilt but also the longing that surfaced. A longing to be near him, to look into his eyes, to touch his cheek as she softly kissed his lips. Everything boiled up in her as she continued to avoid his gaze until one day she decided that she didn't care anymore.
Her new motto became, "fuck them all and what they think". So she decided to be brave for once; she walked up to him in front of the school at lunch and kissed him. Not softly or tenderly but greedily, like she had been waiting forever to have the experience and in a way she had.
A leprechaun could have jumped off a rainbow onto the lawn and no one would have taken any notice. Twenty five percent of the school was there watching them and all of Manhattan probably would know within two minutes thanks to Gossip Girl. It was shocking the way she had done it and even more so when he returned the kiss. It was like they couldn't see the people all around them staring and they probably didn't. In the moment before they broke apart the two became the picture of love and lust intertwined, the longing for a person that ran so deeply you will risk it all and forget about everything else.