My dad and my grandad were blue, so i was born into it. There's pics of me age 3 in city t shirts etc. Dad stopped going before i was born (booo), and never took me (wouldn't win father of the year...) but i finally desperately went to my first city match age 10 in 1985 and have had season tickets with my mum for over 20 years (and paid on the gate on the kippax before that).
So short answer = family, born into it :)