The mirror, James said with a wicked smile and a conspiratorial tone to his voice, will show you the most wonderful things in the world. He'd found it quite by accident one day and is now sharing his knowledge with his best friend.
Sirius listeners with open mouth and wide eyes to James description of what he saw in it, all the glorious and sinful things it hides within its depths. He describes grandeur unthought-of and immoral, beautiful people who throw themselves at him for being him.
What James saw in the mirror was power and sex and merriment and now Sirius stands before it, assuring himself he will see the same.
The picturesque women, the accolades and his friends.
At first he thinks the mirror is showing him just that. There is himself and there is Remus and they look at each other for a moment, a devious smile stretching over both their lips and Sirius, the one watching them echoes it, wondering what mischief they will come up with.
Then his mirror self steps forward, one graceful, elegant motion, and kisses the mirror Remus.
He cannot look, cannot watch them together because the mirror knows everything, knows what Sirius desires more than the cocky, swaggering image he puts on portrays. It knows Remus, knows how he wants him and Sirius cannot watch that, cannot watch this shadow of himself have Remus when he cannot.
So he leaves and as he goes he brushes past Remus, the next person James has divulged his secret too, waiting and impatient to see inside.
Sirius wants to warn him, wants to stop him from looking, wants to kiss him and keep him but he knows he can do none of those things –he only looks on numbly as Remus goes inside, ready and excited to confront himself.
Sirius leans his head against the door, listening, and hears a slight intake of breath, a whimper from within. He does not dwell upon what it might be Remus can see, he does not want to know.
He only whispers "Therefore, Remus, question your desires." And departs from better climes.