We Must Be Killers (a recently triggered passion)
There can be no doubt that the most recent upset in Sirius’ life is the death of his friend Peter. The night that he found out Emmeline was involved, he might have gone and hunted her down if he hadn’t had James there to stop him, if he hadn’t been weighed down with grief. Although he can talk about the incident now without become a white hot ball of anger, the subject of Emmeline and her role in Peter’s death (her role being his actual death) still makes his blood boil. He has a kind of myopic focus when it comes to revenge and if he didn’t have James (and now Remus) to point him in the direction of other things, it might be the only thing he thinks about.
Hold On to What You Believe (a fanatic passion)
Sirius is a true believer of making and following your own destiny. This should be clear from the fact that he had his own soulmate mark burnt off when he was barely ten. Orion and Walburga Black were true soulmates, not faked like his cousin Narcissa's, but Sirius never saw any real love between them. He was convinced from a young age that the entire thing was some kind of cosmic joke, that the names that everyone wore on their skin meant nothing in the grand scheme of things. And he does still truly believe that, even though his belief has been tested more than once.
Seeing the word Black on James Potter's forehead throughout his schooling years was enough to shake the solid ground he'd been standing on since his childhood, enough to make him think that the marks could mean something but only if that means it's me. A second incident was the marriage of his cousin Andromeda to her soulmate. Sirius had almost been convinced that Edward Tonks was the perfect man for Andromeda, a muggleborn to take her away from the Black family, but nothing about that went according to Sirius' wishes. He now firmly believes that the soulmate marks shouldn't dictate a damn thing about your life -- but that doesn't mean that he isn't curious.
These Are Your Friends (a longterm passion)
One subject that Sirius has never wavered on is his friends and his love for them. While he didn't initially warm to anyone but James (who had to convince him that it would be worthwhile to befriend both Remus and Peter, and thank merlin he did) once he warmed up to the other two there was no separating them. Sirius might have occasionally showed preference but if he was ever asked, he'd tell people that all three of his best friends were his true soulmates, not whatever name had been written on his arm. Sirius would die for any of them (would have died, if it had meant saving Peter) and will do anything to keep the friends that are left.