This usually occurs when we speak or smile, when the muscle is contracting. For some people, the dimples are always there. This occurs when the muscle is too short.
This is a condition that is extremely common among children. As they get older the dimples usually begin to disappear.
For those whose dimples don't go away, they can take solace in the fact that surveys show that people find dimples to be more attractive. In rare cases, some people have a shortened zygomaticus major on one side and not the other, leading to only one dimple.