Well, it all starts with Sonic and Knuckles having a quick one behind the shed after the episode where they get the stuff back in the thingamabob. Tails, we thought, was a happy bitailed fox, but somehow we also knew he dreamed to become a hedgehog one day. Or at least a porcupine or a pig with bad hair. Witnessing the scene broke something in him and he fully realised that he could not be happy and make another spiky animal happy with his physique (which was not necessarily true!)