Teens Tend to Match Their Friends' Weight
If you can't lose weight, look to your friends—they may influence your waistline. Researchers who studied 1,751 students from two high schools learned that teens tend to match the average weight of their friends. So teens who were thinner than their friends tended to gain weight and those who were heavier than their friends had a tendency to lose weight. The study doesn't prove cause and effect, however.
Source: PLoS ONE, June 2012