Why would someone choose to live a life full of hatred?

Because of their immaturity and stupidity.

I agree with DigitalGirl, and others who've said it's a kind of defence mechanism. Just another way of keeping people who might hurt you from getting too close, maybe?

I think part of the reason might be fear - whether fear of success, fear of failure, fear of being hurt, fear of expectations, etc. etc. I agree with the premise that it takes more courage to be an optimist than a pessimist. I also agree with what StudentofLife said above, to a degree we each design our lives. That is why some people whine over trifles - and keep getting them - and why others (to whom life may have dealt some really hard knocks) do well & thrive. I choose to look up, keep swimming, smile, etc - knowing that the courage to persevere and have hope usually pays off :-)

Defense mechanism. Sometimes, people think that not letting go of the anger, or thinking of resentment or revenge would make them strong. Some think that forgiving is a sign of weakness. And some, i guess, are just still hurting over things of the past so they can't let go of the pain and anger.