I guess it's mostly geography.  I was born in Toronto, and remember going to an Leafs game at the Gardens when I was about 5 or 6, then their playoff runs in '93 and '94 sealed it for me.  It's really tough being the only Leafs fan in a family full of mostly Sens or Habs fans, family gatherings with my dad's side of the family are hell.  I live closer to Ottawa now, and cheered for the Senators a bit when they first got good and the Leafs were shit, but once the rivalry started to heat up, that ended. 
  
The Jays winning the World Series in the early '90s made it easy to cheer for them, I also remember going to a game with my dad when I was pretty young (maybe 7).  I don't think I got really invested into the team until the late '90s, though, probably after they signed Clemens, that got me really excited.