It's not a classified DSM diagnosis, therefore, it's not a neurological condition or disorder, as it doesn't really affect a person in a substanstial way. It's more of a genetic trait than anything else. Most people grow up with it, thinking everyone has it, but it's very very uncommon. It may run in families- for example, no one in my immediate family has it, but my cousin does.

Different people can have different degrees of types of it, and yes, sometimes it can develop after a stroke or a major accident. Because most people enjoy having it, it's not really an issue that people are actively trying to cure, or even detect.