I've always taken a leader role in group activities my whole life, and Tank is generally the default leader of groups in MMOs. That's how I started tanking.
Actually, it was because our tank in FFXI (when I was playing Monk) sucked really, really bad and I thought to myself "I can do way better than that". So I switched to Warrior to unlock PLD and then mained PLD through Ziliart. I did eventually tire of it and switched to RDM and later Corsair, but I still tanked whenever necessary.
I like playing all roles and always have at least one job I maintain in every role, but I generally enjoy tanking the best as it allows me to lead and control while also keeping party members out of harms way. Instant or near instant queues are pretty nice too. ^_^
