I use open source because I like having the ability to learn how it works. If I can't know how a program works, I can't have control over it. It is not my tool, I am its user.
I also just like learning how technology works in general, and non-free software is generally difficult to share the source code looking from the perspective of a company. Also in my experience, open source software is generally a more fun experience when using it. It gives me a sense of freedom in an increasingly restrictive world.