I love both. I have been surfing for the last 11 years of my life (I'm 21) and soaring for the last 2.5 years. They have a lot of similarities, as many people have already mentioned: dependence on weather, solitary activities but with a community atmosphere...
One of the largest similarities, in my opinion, is the moment by moment decision making that is required for both. When riding a wave, or even when sitting and waiting for a wave, you are always making little decisions that have large consequences. Whether they be not getting the next wave, or falling on your next turn. Likewise, during any XC flight, there are lots of little decisions that again have large consequences. Whether they be skipping a week thermal or deciding to head in a specific direction. And they again have very large consequences. Obviously, they have their differences but I think that in many ways they are quite similar. Heck even rip currents that are often used by surfers to get back out to the takeoff spot act and look quite a lot like thermals do if they were 2 dimensional.