Where do dimples come from? Why doesn't everybody have them?

Dimples are a dent (usually in the face around the smile, but can be anywhere) caused by a shortened muscle. They are a dominant genetic trait, and people find them pleasing, but it's not clear whether they select for them or not. They sometimes disappear as we age due to weakened muscles.
Not all people have them because they are a dominant trait, which means that both parents have to carry the gene.