An RTW usually starts out as a wild dream, then it sounds like something fun to do, and finally you have justified and reinforced it with actual reasons.
Setting certain goals for yourself will give you a sense of purpose, if you need one. This is not meant to be too serious in the beginning, just a way to help you better determine your destinations from the endless number of places to see and things to do. Your goal may be to experience the most fascinating and unusual places in the world, or just see those romantic places you thought you would never get to.
Do you want to move slower, stay close to the ground, get off the beaten path, or do you want to fly from city to city sightseeing? Do you want to visit 100 cities at a relaxed pace, or race through the highlights of 200 cities under the effects of jet-lag? Trying to do too much can exhaust you and end up defeating the purpose of the trip. Going slower will lend itself to different experiences, such as meeting more people and learning more about the cultures.
"People travel for many different reasons, with many different interests, in many different ways. It is quite important in the planning, especially if you are travelling with a companion, to get some things established up front. What (if anything) do you want to achieve by this trip? For most this is quite simply to satisfy curiosity, see things while you have the means and energy, take the opportunity to broaden your knowledge of people, places, cultures etc. For others there is a more specific goal -- to seek employment abroad, or to gather material for a book or article, or photography, or conquering mountains, or diving … It is good to have similar goals to that of your companion, or at least an understanding. Then even those whose goals are fairly vague, will probably have a bias towards a certain type of travel -- some prefer cities, museums, galleries, shops, nightlife; others are looking for remote wilderness with few amenities; some are looking to trek and camp; some have interest in natural wonders, some in history, some in places; some want to participate in activities (e.g. diving, bungy jumping, white water rafting, etc.)." <Chris Finlayson>