Just them being cute in 6th year. Everyone knows their relationship.

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It was pretty obvious that Sirius Black and Remus Lupin happen to be the cutest couple in Hogwarts. Sure Lily Evans and James Potter were cute but they weren't Sirius Black and Remus Lupin cute. Even the professors could agree that these two had something they had never seen before. 

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"And so we see the couple in their natural habitat, the common room. You can experience exactly the tension that Peter and James deal with every moment of their days," James commentated as he walked into the common room from the dormitory. 

Sirius threw his head back in laughter. James had been doing this lately, acting like some documentary commentator ever since Lily introduced him to animal documentaries. Remus stayed silent as he watched James huff. "Well it's true, every Gryffindor has seen you two snogging on that couch," 

James crossed his arms as he plopped onto the couch across from them. "How are you two so perfect," 

It was Remus's turn to burst out laughing. "It's not rocket science," Though to James, who although was always trying to whoo his girlfriend, it certainly was. 

"Come on, there's a reason we call you two the cutest couple on Hogwarts," James pushed. "There has to be a secret," 

Sirius shrugged. "There isn't, I just show him how I feel, which is that I love him," 

Remus blushed and looked away. "I think Padfoot did a good job summing it," 

"And there you go, being the cutest couple as usual," James said as he threw his arms up in the air.