Why DDOS us when there are infinite other IMHO places more deserving

Other than plain maliciousness, does it come down to numbers e.g. I can shut down these sites because I have control of more bots than they have server capacity as opposed to something like citicorp where I don't and can have no real effect except for keeping a few ticked off folks working overtime on christmas day?
Why? I want to know why.
And if it is about 'bots', how do I know if I'm not one of them?