There can be many reasons attributed to why people suddenly decide that the life they have is simply not enough and they want to go all 'world domination' on our ass's and basically take over the world:
~Boredom- not one person can say they haven't done something reckless or dangerous just to relieve the boredom, I know that I used to climb trees when bored and nearly fall out of them (Not on purpose)
~Bitterness/vengeance - some people or evil bad guys think that society owes them something, for either shunning them, being bullied in highschool because they are prematurely bald (Dr evil, austin powers) or for simply not recognizing their brilliance, which leads them to believe that they will get noticed if they simply try to take over the world
~For the thrill _ again this relates back to the whole boredom thing, some times the thrill is to relieve boredom or for the adrenaline rush that may or may not come with trying to take over the world and having to be sneaky about it.
~ Power - It creates dictators, wars, epic struggles between nations and people for power over people, to control them, that feeling when you realise noone can touch you, you are epic, legendary but of course in films they always fail, so that would be most likely a dream sequence
~ Nuture - In Dr evil's case, he was raised by belgium people to be evil and hate everything so it can be down to how the person was raised or what morals and beliefs they were brought up with and were led to believe where the only rights in this world.
Or in some cases they can be just plain psycho...
Or you can just ignore the sixteen year old who has just typed this (me) and just go about your day instead