I have been asked this question recently in an interview and was very dis-satisfied with my answer. I thought another short attempt could be worthwhile.
I am utterly interested and fascinated by the notion of free software, though I do not like programming at all.
I view software as a language, perhaps a derivative of mathematics. Just as with maths, I cannot think that one should release a result (a working piece of software), while also restricting access to the proof (the source code) and forbidding inspection and modification.
Proprietary software, such as Microsoft Windows, is an aberration to me. It is pretending that a mere compilation of thoughts, intangible processes and ideas, is like hardware: a manufactured box, that can be stolen, broken, and merely re-produced, instead of copied.
curiosity and bemusement
I generally deeply distrust society and human group behavior; yet free software shakes this suspicion.
Indeed, tell me that you will re-invent copyright, seriously question the very notions of ownership and intellectual property, and shuffle traditional authority, governance, participation and decision making establishments: I will smile back and sigh.
Yet, it works. The success of Wikipedia, or the completeness of Linux, will astonish anyone who has read a good history book. I view this as the single most interesting social experiment in our history; I observe its progress with interest, and strive to participate.
We are awfully bad at mostly everything we do – just ask a child.
I put a lot of hope in free software, convinced that it is necessary for a free society, an indispensable ingredient for freedom of expression, and, by extension, a solid frame for individual reflection and responsibility. In an amusing way, thought-enabling, hope-powered maths ;-)