I like making computers do useful things, but they aren't always very cooperative, and sometimes we disagree about what's right.

Having source code helps me understand things from the computer's point of view.

Having open source tools and infrastructure to compile it helps me make my system behave better.

Being able to share my changes with the rest of the world helps me and others learn the wisdom of those changes.

All these things can still happen with closed source, but the process takes longer.

tl;dr - I'm only going to live for so long, so open source lets me live a more productive and fulfilling life.