Ryan Tubbs, the mayor of Celina, on the town's square.Alcynna Lloyd/Business InsiderCelina, Texas is the fastest-growing city over 20,000 people in the US.Mayor Ryan Tubbs said two things set Celina apart. The first is its tight-knit community.The city is also more affordable than parts of California and Texas many people are moving from.America's fastest-growing city isn't what you might expect.It's not a big metropolis where movies are set like New York or San Franciso.