It was a surprisingly easy question to answer for Sirius.
"Because you're a bloke that's a bit on the reserved side. You're able to keep up with me. You banter back, you tolerate my penchant for mischief, and you even join in when the fun isn't too recklessly stupid," Sirius poured it all out, and Remus would probably remember how he'd described his ideal candidate in the same terms when they were gazing up at his Ziggy Stardust poster. But there was more.
"You don't know how fetching you are, and somehow that's always turned me on more. You're just– just bloody amazing, all right? You're selfless and sensible and a lot of things I'm not. You're even willing to see my lousy poetry and that's why I'm not giving up on you just yet."
Sirius had never had so many reasons to fancy someone. Usually it boiled down to looks and that was that.