Eh, don't feel bad, it's a good thing to say please and thank you.
It's just that on dA, some people don't want to be thanked for faving someones works and giving a llama as they would appreciate more meaningful comments. When they fave they simply wants to see more artwork of that person.
And this stamp-button doesn't cover much, since there is some thank you's that will always be appreciated, like when a user gives you out a subscription or points because of how much they appreciates you.