If you live in an apartment, expect your rent to rise again.Average U.S. rent has climbed 14 percent to $1,124, since 2010.That’s according to Commercial Property Tracker Reis Inc.Greenville isn’t that high, but Alex Crigler with C. Dan Joyner Realtors says it’s pretty close.Zachary Pfaff rents an apartment on Pelham Road in Greenville and said his rent has gone up $100 in the last year."In the past I thought renting would be more affordable, but it's just not the case anymore,” said Pfaff.Crigler said the increase is partly due to a spike in the number of people who live here.In 2014, Greenville ranked ninth in the nation for population growth.“Right now we have so many people moving into Greenville for job opportunities that have just graduated, that are single or recently married, and everyone wants to be downtown,” said Crigler.Crigler also said the number of people who graduate from renting to owning a home is decreasing.  And even some who buy a home don't keep it.‘We've seen a lot of families when kids go off to college, empty-nesters, they want to go back downtown.  They want to be back in an apartment,” said Crigler.And Crigler said that, quite simply, apartment living is trendy.“The closer you get to downtown the higher the demand.  People want to be within walking distance, they want to be right there on those sidewalks,” said Crigler.Crigler also said the high demand means renters are less likely to be able to negotiate with landlords for a lower price.The three cities with the biggest annual increase in rent in January were Denver, Oakland, California, and San Francisco.