This is like the biggest thing my group here drills new bitcoin users with.... if you don't control your private keys then you don't control your account. The Bitcoin network has never been hacked. The only current feasible way to hack someones bitcoin is to get access to those keys. With most exchanges you do not have your private keys. So you do not own those bitcoins directly. They are owning them on your behalf.... if someone hacks your computer and is able to get your keys or gets and exchanges keys... they can take that money... if you pick a key... instead of a computer picking it ... if that key is guessable... your money can be taken... exchanges only own the wallet on your behalf on the network... this is why you should make sure your keys to your money are safe... I keep mine encrypted out of reach for my personal wallets