An: takes place during season 5.
Elena watches Stefan. Damon watches Katherine. Katherine watches Stefan. Stefan watches Katherine.
Elena knows that she shouldn't be watching Stefan. She had slept with Damon the day after they broke up. She chose his brother over him and then lied about being with Damon but almost kissed Stefan while he had amnesia. Yet she couldn't stop the jealousy that crawled into the pit of her stomach when she saw him watching Katherine with that soft look on his face. In the darkest recesses of her mind, sometimes Elena wished Stefan would look at her that way, like he used to. Strange how the tables had turned. Stefan used to watch her, but now he watched Katherine Pierce and Elena was left watching Stefan wondering 'what if'.
Damon knows that he shouldn't be watching Katherine. Elena was the one he's in a relationship with, the one he fought so hard for, he should have his eyes on her and only her but he couldn't stop watching Katherine. He noticed every little move that she made. He watched as she peered over her book to catch Stefan's eye. Damon watched Stefan look at her intensely for a moment before turning away dismissively. Damon watched Katherine glance back down at her book hurt by Stefan's rejection. After a couple of moments Damon sees Stefan look over at Katherine when she isn't watching him; his eyes are focused completely on her, like she is his world. For months Damon had been watching Katherine watch Stefan, and now Stefan was watching Katherine back. What Damon wouldn't give for Katherine to watch him again like she used to. Now Damon watched her, regret deep in his chest.
Katherine knows that she shouldn't be watching Stefan. She had ended their "fling" because he was clearly still in love with Elena and she was human. Blaming her decision for them to separate on the cure seemed like the easiest way to end everything. Katherine's own crush on Damon had somewhat fizzled out when Damon had fed her to Silas for his own selfish purposes, so now to Katherine, Damon was just another boy who used her, but Stefan; he was a man she had stupidly given up in a moment of weakness when she decided that she didn't deserve to be Stefan's consolation prize. Katherine was just too proud to admit that she had made a mistake letting him go, so instead, she watched him.
Stefan knows that he shouldn't be watching Katherine. He should be keeping his eyes on Elena, everyone expected him too. Elena was the reason he stayed in Mystic Falls in the first place after all. But there was something about strawberry scented lip gloss, skin tight jeans and stiletto heels that made Stefan unable to stop staring at Katherine. Some days Stefan imagines telling Katherine everything he feels about her. How she had always been the light to his darkness. Somehow managing to keep a vicious monster inside him in check, even though she's human. She challenged him to fight to be something more then that monster that gives into blood. Sometimes he imagines her laughing at their stupidity and saying that she loves him back. It's one of his favorite daydreams. But Stefan knows he will never tell her. He's an asshole who's supposed to be in love with his brother's girl; he wouldn't wish himself on anyone. So Stefan keeps his mouth shut. Katherine Pierce is human and might be going somewhere in this world, and Stefan won't hold her back, he won't. Stefan was happy to stand back and watch her shine.
Stefan watches Katherine. Katherine watches Stefan. Damon watches Katherine. Elena watches Stefan.