Foaming is a huge reward, said Sinclair, the brand manager. Shampoo doesnt have to foam, but we add foaming chemicals because people expect it each time they wash their hair. Same thing with laundry detergent. And toothpastenow every company adds sodium laureth sulfate to make toothpaste foam more. Theres no cleaning benefit, but people feel better when theres a bunch of suds around their mouth. Once the customer starts expecting that foam, the habit starts growing.

The Power of Habit by Charles Duhigg