Starting a new song is kind of like when you first fall in love. Everything goes along effortlessly and all is right with the world. But then as the weeks (days? minutes? This is a metaphor after all) go by, you start to notice the little imperfections. Not enough to run away or abandon her (we'll make it a her for now), but maybe just entertain the idea that there might be someone out there that is more understanding. More spontaneous...
But really it's just a fairytale - Really, what you're looking for is that spark of newness that your brain get's all dopamine-y about - which scientists believe is just a way to get the procreation thing going and further the species. But really we're just fooling ourselves if we really believe that the new song is going to be any different than the one you're with now. There's always going to be rough spots and spats and things you wish you could have done a little differently if you thought about it.
It is a rare occasion when a song just spills forth with little or no effort, and all artists tend to agree on this. Those moments are precious and few and you're going to be really lucky if she doesn't turn into a pumpkin before you're done. Then of course you have to spend the rest the week finding the foot that fits the slipper, or more often than not, cramming the damn thing in there until it turns blue.
