What if I'm writing a program for fun and don't want to deal with a bunch of faggots trying to embed themselves in its development?
What if I accept patches and some well-meaning asshole starts implementing all of the "fun stuff" I wanted to enjoy doing in small pieces to keep myself motivated as I work on the actual hard problems?
What if they actually do take on the hard problems but do a shit job of it? Either I accept the patch and now have a bunch of shit code in my codebase that I don't understand, or I reject the patch and piss the guy off.
What if I don't accept patches, then take a break for a bit, and when I come back, I find that someone else has forked my project and taken half my userbase away?
What if I'm aware of this risk and instead overcommit myself and become unhealthily obsessed with my project so as to ensure that it will be superior to any fork, but eventually it burns me out and I give up on it?
What if even then, some faggot still forks the project, implements a new feature in a terrible way that shits all over the codebase, but my mouthbreathing users all jump ship anyway?
What if some parasite starts submitting trivial changes to pad his resume?
Even worse, what if he just copies the source, changes some names and formatting, and passes it off as his original creation?
What if some feminist sicks an internet mob on me because I used "he" somewhere in the comments?
What if I start a dev mailing list and have to deal with a bunch of well meaning retards asking 20 questions a day and saying "wow I wish I could help you but I'm not a good programmer :)" in return?
Fuck all of these parasites. The only reason I share anything is for the resume.