This is why I like Microsoft:
* You get to impact the lives of millions and millions of people. I find this really cool. I can't wait for Office 15 to come out and be able to see people in my class or on the bus or whatever using features I made. You feel a big (and mixed) sense of responsibility and pride. What we are doing really does matter. If you fuck up, millions of people are going to be affected. If you do an amazing job, the same thing happens but in a positive way. Just think about that for a second.
* Microsoft is virtually involved in every market. Want to make an OS? Go work on Windows. Now you're working on a product with ~85% marketshare. That's pretty cool right? Want a change? Want to see what it's like to work on a product that is the underdog? Go have some fun in Windows Phone. After that, if you want to give search engines a try go ahead and work at Bing. If that doesn't work out, you literally have so many other options (Xbox, Azure, Surface, Visual Studio, Office, IE, etc.)
* You can really feel that you're in an atmosphere of extremely bright and intelligent people (for the most part). This keeps you extremely motivated in case for whatever reason the above things aren't enough.
* While this may seem silly and very minor compared to the above points, it's pretty fracking (sorry been watching BSG) cool to use software before it gets revealed or released to the masses. Knowing secrets is fun :)