This is a fascinating question which I have been frequently been asked, especially by Mrs P when she looks at our bank statement.
Having gone back to flying after a break of 23 years I have thought often about it and, for me, there are many reasons. My second incarnation as a pilot has provided me with a huge buzz, first solo and especially QXC will remain fixed as delightful memories.
The paramount reason why I fly, however, is that I love controlling machines. I love driving (just as well because I do a lot), I love helming boats and flying aeroplanes.
The smoother and more effortless the operation the happier I am. I find it strangely satisfying. Because it's three dimensional, flying has got to be ultimate, and the best moment in any flight has got to be the final turn for landing properly executed. If I could afford it I would fly a jet because this provides the ultimate in smooth control - I once did military circuits in a JP5 and I found this particularly satisfying.
Looking at the ground, clouds and the satisfaction of a well executed XC are also reasons to fly. Conversely, a bad flight makes me think of giving up, until the next good one.
Other reasons why I fly are that I like being a member of a "club" with like minded people, which is also a good reason to be on Pprune.