fat

why is it so easy to gain weight while so difficult to lose?

Because our body is one giant magnet for bad, unhealthy food.

Update:
"If your friends and family get fat, chances are you will too, researchers report in a startling new study that suggests obesity is "socially contagious" and can spread easily from person to person."

Ref: San Diego researchers find obesity can spread in social circles

Does it mean we will have to disown all friends/family above a certain BMI to lose weight?
The study says no: "Despite their findings, the researchers said people should not sever their relationships."