Do you notice whether you are more at home at sea-level, or the mountains?
I think that we fall into categories of altitude preference just as we prefer cooler or warmer temperatures. I've lived for at least two weeks at all sorts of altitudes, from 50 feet (Brooklyn) to 6,000 feet above sea level (Switzerland). To me, things seem much better between 1,000 - 1,500 feet. Around 2,000 feet the environment becomes a bit too marginal for my tastes; life seems like a more dicey proposition. By the time I reach 5,000 feet I'm thinking in survival mode, but I am also enraptured by the exquisite purity of the air and am thrilled to have a specifically limited amount of time to enjoy it. My father goes hiking a lot at this altitude, and then comes back to sea level here and declares the air "too rich" -- his lungs aren't used to all this oxygen and humidity.
What about you?