Once a sensei advised me not to smile or laugh so that I might have more credible aikido. But I have failed to follow his advice, so far. i truly believe aikido should be practiced "in a vibrant and joyful manner." Don't get me wrong about not taking practice seriously, I DO. I practice like there's going to be no tomorrow. Just the gift of practice time/mat time alone is enough to make me smile. What more if I had a really good technique applied to me. Wahah
And yes, let off a few giggles every now and then..