"You bet." I do that too.

It's easier than saying "Well, it's certainly gracious of you to indicate your gratitude for the services I have just rendered on your behalf, and I wish to acknowledge your thanks in a totally satisfactory and reciprocal manner, which entails me thanking you...for having just thanked me. If I do that, however, I will expect you to thank me again...for thanking you for thanking me."

And so on...

I find that if you pull this routine on people too often, they usually stop thanking you altogether and may even get physically violent or verbally abusive.

It's a better bet to simply say: "You bet!"

- LH