Costco (NASDAQ:COST) shares are up about 30% this year, and are sitting near a decade-high valuation of 27 times trailing earnings. So is it time to start thinking about selling this retailer? 

In the video below, Fool contributor Demitrios Kalogeropoulos says no, and outlines three reasons that he's happy to be holding shares right now. First, Costco's recent performance shows that it's still the leader in its space, performing much better than rivals like Wal-Mart (NYSE:WMT) and Target (NYSE:TGT). Second, the company's membership base continues to provide increasing fee income, with members renewing at even higher rates lately. And finally, the warehouse retailer is planning a banner year of new store additions in 2014, which will lay the groundwork for some big sales gains down the line.