So! Susan's experience with magic prior to the Carnival was... a bit skewed. He's met a lot of people who are very serious about it, due to mostly knowing a shit-ton of druids who revere nature itself and are crazy about it, and the few other magic users he's met before are either people trying to kill him, or people who felt really serious or dull - teachers at his attempt at magic school, other adventurers trying to fix problems, etc.

So when he met Strange, that was different. Not that he thinks Strange isn't serious about magic, but he doesn't act serious, and that right there made Susan instantly like him. He really is interested in learning more and getting better, and so when he met someone who actually was interesting and fun who also could cast magic, he was super excited.

It helps that Strange doesn't really talk down to him like a kid, either, or tell him what to do or when his ideas might be crazy. With how he grew up, he absolutely hates people treating him like a kid, and so having another adult around who actually treated him like a person definitely put Strange on Susan's list of favorite people.

Overall, then, Strange is one of a very select few people here in the Carnival that Susan actually both likes to talk to and trusts, that he would actually have a serious conversation with or talk about his problems with. Maybe that's not such a good idea, who knows, but Susan doesn't have a lot of people like that, so maybe it's alright.

And also he thinks it's awesome that Strange wants to teach him magic? That's actually the best part in his opinion. Magic is the best in his opinion.