Altruism?
Sure! I want to share knowledge. I want to contribute to make this world a better place. Makes me feel that I am a valuable contributor to the community. Feels good, so I guess this is a selfish motivation. Can selfish objectives be considered altruistic? I don’t know…
Sense of community
Again, being a part of something. Interacting with other people in a positive setting. Feels good.
Debt
Someone made an effort to create this software package, then made the product freely available. I benefit from that, and want to pay back by contributing to the product in any way I can.
Learning.
By searching for solutions to other people’s issues, I learn new things.
When my efforts, delivered “to the best of my knowledge”, are contested, I sometimes stand corrected. In this situation, the “best of my knowledge” is augmented. I learn even more.
When I receive comments, I learn a lot from those too. About software, language, understanding users’ situations, typography and graphical design or personal relationships and interaction.
So in the end it seems that my best reason is in connection to my own learning. Not sure that it is the most important motivation, but it sure is one that counts.