I know that %95 of all dietsing fail, so basically anyone who's unhappy with their wiehgt is on a diet forever, but nothing changes much.
I would say %75 of the population want to lose weight, though quite a few of them don't have to. Probably no more than %1 want to gain weight, but this data excludes people who should want to gain weight but are too sick to realize this.
This is an amazing video about losing weight: