For the last few years, I've had the growing understanding of how the internet promotes GAS.
Think about it. We now have instant access to photographs and camera information from all around the world, and the ability to buy gear online. I used to think that having more than a couple of cameras was excessive, but as I looked at more and more flickr accounts, the more I saw people with four, five, ten cameras or more! When I only had a Canon S45, I looked in envy at the people with a Rebel/Xt, and even when I had a 30D, I used to wonder whether some day, I would go full frame as well. That day came and went, and I have the 5D Mark II, Leica M9, and a swag of lenses for both systems.
In the last decade, more and more people have gone multi-system. For some it is Nikon, Leica and Olympus. For others it is Sony Alpha, NEX and a few pocket cameras. But our ability to see others with truckloads of gear has a normative effect. What was once the exception is increasingly becoming the rule, not just because we can buy it, but because we see other people doing it, and it no longer seems excessive.
My GAS attack of 2012 began with a walk through the city in the winter. Cold and wet, I huddled under an umbrella while alternately trying to shoot with the M9 and hiding it from the rain. The OM-D was in full marketing swing, and the thought of a compact, weatherproofed body and lens was very enticing. So I got into m43, too. OM-D, Sigma 19, 30; Panasonic 14/2.5 and 12-35/2.8; Olympus 45 and 75. And this gave me a complete set for day to day shooting.
This year, as my work has become increasingly about video production, I've got a Panasonic GH3 and a few more lenses, not to mention a dolly slider, new tripod, light stand etc. While I can justify this for work, it also reflects the kind of gear I might have bought anyway, had I not been in this industry. The 35-100 and 7-14 are amazing on the OM-D and GH3!
But everything I buy now must answer the question: "where does this fit in the system, and how does it help me improve my photography and make more money?" Because now that I am fortunate enough to do what I love to make a living and a life, everything has to fit.