Some dogs become less social with strangers as they mature. Some also go through "late fear periods" as "teenagers" (when they become sexually mature but not emotionally mature yet- roughly 6-18 months old.)

If the change in her behavior seems extreme or came up very suddenly, then yes, I'd suggest a vet visit. If it's more subtle, I'd try to get her over her fear with controlled desensitization (gradually increasing her exposure to strangers, using treats or playtime to make them not scary.)

Bruno answered on 11/14/11. Helpful? / 0