Mostly because I was retired and bored. I have been "Watch" weather since a kid and lived on a lake. We never liked being trapped out in a boat when the storms blew in.
I started by doing on line courses via Comet systems then local classes. I go every spring to the classes provided by our local NWS. Staying current with their training and procedures is a must.
I don't remember when to tell the truth, late 1990's. I guess. I figured as long as I was out shooting it, I might as well do some good. I took the classes at the NWS office in Sterling, VA They stopped holding classes there after 9-11 and now hold them all over the place. I don't remember how I found out about them, I think it was from the old news letters Tim Marshall send out in the early days of Storm Track.
I became one officially in March of 2017, I had always been intrigued by the power of tornadoes and severe thunderstorms since I was a preteen, but after living through the Tornado outbreak May 31,1985 in NE Ohio and western PA, I wanted to do something to save lives, besides I was inspired by the guy that stood on the roof that night in Newton Falls Ohio and saw it coming in, and warned everyone in that town with nothing but a two way radio, I thought "man that guy is a hero" and I wanted to do something big like that
I have lived in Tornado Alley all my life and enjoy watching/photographing weather.
I got into amateur radio to enhance my off road communications abilities, which turned into a hobby.
About 2 or 3 years ago I heard about the Skywarn classes through the local radio club as they were hosting one of the classes and as John said, since I was already out there I might as well report on it.
Now I take them every year as a good refresher.