In no particular order, but for many reasons.
1. You have more stuff than you think.
2. Whatever amount of time you allot for moving your stuff multiply by three to get the actual time.
3. You will find that "thing" you thought you lost a year ago, and you'll end up throwing it out anyway.
4. Because most of your stuff isn't square, most of it will not fit nicely into packing boxes.
5. "And on the 8th day, God created the hand-truck, and it was good."
6. National Geographic magazines present a particularly difficult quandary.
7. You actually, really do own 37 plastic storage containers. You just didn't know that until now.
8. You find more lost silverware that you never knew were lost.
9. You discover that those lost socks went behind your washing machine to die.
10.You discover that you own more medication than is typically found in the average sub-Saharan hospital.