So when you pack, you're always trying to find all the pieces of something, or all related items, etc., so you can put them in the same box and put on a nice label: Hair Accessories; Little People (the Fisher Price kind!); Napkins.
Then, 15 minutes later, a related item makes an appearance. What! Another hair tie? Where were you 15 minutes ago??
Now, you have a hair tie. But the Hair Accessories box is already sealed, taped, done. You could untape it, put the hair tie in, and retape it... but that seems like too much work for one hair tie. So you put the hair tie in a remotely-related box - Bathroom. You seal up Bathroom and... an hour later... a headband shows up. What! Where WERE you for the last hour and fifteen minutes??
But the latecomer needs to get packed, too. Do you start a box for random items? Do you start a new box for Hair Accessories? Do you just give up and start packing everything in one big mess? Now you're going to have hair accessories in THREE boxes. Gah!
This is the worst part of packing.