The Smaller (& Lighter) YOUR Bag, The Better The Trip
Here you'll find a list of what you really need on a backpacking trip, put together by those who have learned the hard way -- not someone who is trying to sell anything other than the concept of traveling light.
The happiest traveller will be one who can fit their bag/pack under the seat of a bus or take it as hand luggage on a flight. It might seem impossible, especially when first throwing a few things in a bag, but after learning the hard way you'll start taking less and less on every trip.
In reality, you might have to learn most of these lessons yourself but once you collapse under the weight of your corpse-sized bag, we'll try not to say "I told you so".