In fact the do not they all start with C as in CYYZ which makes much more sence then P or K as in PHNL and KORD like they have in the US. ANyway this topic has been dicussed to death, so have a look in the history, basically it is a throwback from the railroad industry in Canada, all cities got a three letter code, starting with Y WHy?? WHy not? Toronto's is YTO, vancouver is YVR, montreal is YML.
"Clive Beddoe says he favours competition, but his actions do not support that idea." Robert Milton - CEO Air Canada