In U.S. English, "Here you go" means "I'm giving you what you asked for." In tone, it is informal, cheerful, and friendly.
When someone performs any kind of service, even a small one, it is polite to say "Thank you." Whenever someone says "Thank you" to you, it is polite to reply "You're welcome." As Jmat says, this is a standard pattern, almost a ritual. In the U.S. the best and most common response to "Thank you" is "You're welcome."