Because he is both stronger/more successful /s
It's because Buggy is Rocks' son, and has natural charisma.
Buggy has always nerfed himself, because he looked down on himself due to Shanks being so strong, and Shanks being Roger's protege, learning Kamusari. There's a reason Oda hasn't shown us that scene yet: Shanks must have been thrilled, but, that was probably the most where Buggy realized "I'm number 2".
That's never how Roger/Shanks viewed it, but, that's always how Buggy felt, like he wasn't good enough.
Now that Buggy is committed to becoming strong, after learning Shanks was going for the One Piece, we'll see his true power.
Sadly, I believe Buggy will legitimately come to hate Shanks when he finds out Shanks was a Celestial Dragon.
I think they're going to have a very tragic character arc, overall.