For me it is a lot of things. For one, it is a simultaneous mastery of several disciplines. When I fly, it is all about the flying, all the other clutter in my mind is set aside, and that is well, refreshing and cleansing. Then there is the beauty, you see things from a perspective that few others see things. How many people out there can skim across the tops of clouds that are just a few feet away, how many have seen the 'glory' on the clouds right below them (a circular rainbow that circles your shadow on the clouds). There is also the risk and the skill required to minimize the risk, which leaves you with a good feeling when you are successful, and gets a lot of adrenaline pumping when you are not so successful. There's also the freedom (at least here in the US for now) to go more or less where you please how you please. There's nothing else quite like it.
MrsGGB calls it "the disease", with one of the most recognizable symptoms being a propensity to drop what you are doing and look up in the sky when the unmistakable sound of an airplane engine is overhead.
