Why is it so hard to lose weight?

I am on mission to research as much as I can about weight loss. Here are some articles I found helpful.

http://www.heractivelife.com/women/comment/why-its-hard-to-lose-weight/

http://psychology.suite101.com/article.cfm/how_hunger_works

http://seattletimes.nwsource.com/html/pacificnw09102006/2003248131_pacificploser10.html
http://www.latimes.com/features/health/la-he-regain2-2008jun02,1,3711514.story

http://www.webmd.com/diet/features/focus-thinking-differently-lose-weight?page=3

http://www.nurturedsoul.com/drop-pounds-and-inches-with-a-positive-attitude/

http://www.nowloss.com/how-to-break-out-of-a-weight-loss-plateau.htm