There are some themes I had never thought in making mail art about, had it not been for my fellow mail artists or pen friends, and their (sometimes) crazy projects. Like platypus, or cats and shoes, or solitude, or teeth, or onions, or zombies, or poppies, or frogs, or horns... Some of these themes were completely unknown for me before I started to make the mail art. So I am obliged to learn a little about. For instance, I should look for a platypus picture on the net (Gods save the net!) before attempting to draw it, and even a poppy one. Yes: I am afraid I am not able to make original drawings: I always copy them. And I still don't know anything about soccer, by the way.
I wonder why I feel attracted by some themes and why other mail art calls let me indifferent. But I guess you have not the answer...
I also wonder why people like platypus, and why people wish to get mail about teeth, or stones, or Shakespeare, or... But I suppose they feel exactly the same surprise when I ask them to send me laughing cows or postcards related to Oz. One half of the world cannot understand the pleasures of the other (Jane Austen dixit).
Anyway: I am terrible grateful to people able to come up freak themes for mail art or odd places to live in (and buy stamps in), because they let me learn a lot of new and amazing things.