My reasons:
+ The iPad was my gateway drug to the iPhone. The iPhone was my gateway drug to the Apple ecosystem. All of the "relearn an OS" cruft I was expecting to hit was greatly, greatly shortened by the experience of being on iOS.
+ Apple is a monoculture in the development communities most of interest to me. Ruby gems would break randomly on Windows due to e.g. using `ls` to get the contents of a directory rather than using any of the stdlib functions. All tutorials start with the assumption you're using homebrew. The final straw for me was when I started needing infrastructure pieces that just wouldn't run on a Windows machine -- Redis was, IIRC, the motivating technology. (MS sponsors a Windows Redis these days. Good call, a few years late. Sorry guys.)
+ I'm no more immune to social pressure than anyone else is, and my peer group was actively mocking me for running Windows since, minimally, five years before I switched.
+ I do a lot of international travel and need a laptop with me. Previously I used "the best Dell makes", generally a Dell Studio, with a big screen and lots of RAM. I affectionately called my Dell "the aircraft carrier", because it was heavy and bulky. My most recent Dell was a replacement for one which, when on a shoulder-mounted laptop bag, swung out an inch too far and caused the power button (side-mounted) to collide with a door frame, breaking it. (I had to turn that machine on by shorting the two wires attached to the power button which now strick from the case!) Every time I got back from the airport I complained bitterly about how heavy the bag was, and was mocked again by friends who ran functionally equivalent computers which seem to weigh about half as much.
+ Might not be relevant to everyone, but highly relevant to me personally: my main development machine also has to play video games. This was a major thing keeping me on Windows, and literally the day before I bought my first Mac, I said "I might get myself a Mac purely for programming, but my main machine has to play games." "Which games?" "Recently, League of Legends and Starcraft 2." "Both are available on Mac." "I'm buying a Mac tomorrow."
+ As Thomas said, every Macbook looks like it has been sculpted out of aluminum. Equivalently powerful PCs are... am I allowed to say ugly? I used a Dell machine made and marketed specifically to artistic professionals -- it's literally called "Studio" -- and even to my very limited standards it seems to lack artistic taste, particularly when it is sitting next to a row full of Macbooks.