Anybody who's in the business of making stuff -- from knitting to cooking to computer programming to writing -- will hopefully have experienced that feeling of satisfaction when the cool idea in your head is finally realised, when you look at your finished jumper or jam roll or java applet or journal entry and think Yep, that's about how I imagined it. It's a tiny click in the back of your mind as a couple of bits of the vast chaotic universe fall into place.
Where cake-decorating differs from most of these things, though, for me, is that it's very hard to imagine doing it purely for my own amusement or satisfaction. I can write for no audience but myself; I can knit garments for myself or just for practice; I can cook to feed myself; but if I'm making a cake, it's usually for a reason -- for a special occasion, for somebody I care about.
(The downside of this, of course, is that it always matters: if I mess it up, I'm not just messing up a design but also an occasion, someone's birthday. In its own small way, it's a big responsibility. I don't know if I'd dare do a cake for a wedding... fortunately, nobody's ever asked me.)
The best thing about cake-decorating, though, for me, is that it's a fundamentally transient art-form. I'm not creating something which has to endure; I'm not going to be mocked in millennia to come by vast and trunkless legs of cake left incongruously standing among the ruins. It's a short-lived beauty, and all the sweeter for it (though 10 oz. of sugar may have something to do with it too). You can't hang on to it: if you keep it, it mutates into something other than a simple pleasure, gathering meaning and moment which it was never meant to have. (Quite literally, you can't have your cake and eat it.) The culmination of the artistic process, for me, is the moment when the knife cuts into the design: that's when the cake has been admired to the full, like a balloon filled to capacity with helium, and that's when I can let it go.
In practical terms, it's a gift that you don't have to keep: it doesn't hang around growing steadily greyer with the dust of years and the residual guilt for friendships half-forgotten. It's a gift that's meant to be used, a gift you have to enjoy. And in metaphorical terms, it's just a big sandcastle, every turret and crenellation carefully constructed in the full knowledge and acceptance that they will be swept away by the tide.
In short, it's like an allegory, but with more raisins in it.