Packing is never fun, there's always the temptation to take more than you need. That's never a good thing, as it makes your bag heavier, stuff is more difficult to find and you're essentially just taking items of clothing along for the ride.
So why do it? Well, it's in our nature to delay decision making, so if we just pack it all and decide what to wear later, our lazy brains are happy. Underwear is different because it's small, so you feel like you can throw in plenty and it won't matter. Plus, putting on a fresh pair of boxers always feels better than fresh pants, or a jumper. Doesn't it?