Actually, I do tend to give a Llama if I browse someone's gallery. Although I must admit I haven't really gotten all this llama business.
The other reason I give a llama is in return, as was your case. But I also admit I don't really expect a llama in return if I give one, it's totally ok if ignored. I understand a lot of members resent being thanked for giving llamas away. And I can understand that the newsfeed can get clogged quite quickly by thank yous.