Well, I was only 5 when Y2K rolled around, so I don't really have a lot of memories from the '90's. Even so, I think Sonic's personality is a soft stereotype of 1990's culture (and by "soft stereotype", that's opposed to a "hard sterotype" which would be something like Wayne's World), and for whatever reason, that just makes me feel comfortable. Maybe it's unconscious nostalgia? I don't know, I don't care. Not always about snappy attitudes and all that; I like how, even though that's games' feel most of the time, there are the rare few games that amazingly pull off a dark and serious vibe. Like Knuckles' Chaotix (one of my favorites), or Shadow The Hedgehog on mute while you put on some \M/ETAL.
There's also the in-game art. I don't know if it's just because I'm autistic, or if anyone can relate to this, but I can find beauty in an artificial landscape, as long as it's made well enough. And that can be applied to sooooooooo many Sonic levels it's not even funny. And the music? Don't even get me started! The only way you could say you don't like the soundtrack to at least 1 Sonic game is if you just didn't like music. Well, that's how it feels to me, anyway. On top of that, some Sonic games feel like works of art on their own. Sonic And The Black Knight is like an interactive feature-length film; so is Sonic Unleashed, if you actually care to get through those Werehog levels.
I've got a few more reasons, but, I don't really want to type it out right now.