1. The human psychology versus the human physiology. The mind can dream; reality is different and often more pessimistic than in theory. Life is messy!
2. Power imbalance - especially with the childrens' brains being ruined by school and its stress, grown-ups being trapped in a job they don't like for money due to a lack of alternatives... This one, and the one above, are hard to fix - and affect a lot of people - so they are indeed the 2 biggest enemies of mankind.