I think that Sirius didn't really know what he wanted either. He had been living with his family that he couldnt' agree with and only knew he didn't want to become them. He hadn't yet figured out what else he valued or wanted. James knew his mind well because he had grown up agreeing with his family and could figure out what he valued in life rather than just know he agreed with his family.
Originally Posted by R_Ravenclaw
Sirius was more reckless because he knew his family wouldn't approve of it. He hated his family so much that he would actually do whatever they didn't approve of.