I want people to use the software I write, and people don't want dependencies in their projects that are gpl because you either need to change your project to gpl or go through a very tedious process of providing object files for the entire project. That's why I use mit for libraries.

However, I don't want to allow companies to just take my finished binaries and resell them as their services, so I use GPL for binaries and apps. They are free to reimplement the app using the libraries, but I at least want them to work for it a little.

Also, I have found it easier to get contributions to projects that are licensed under mit than gpl, though this might just be insufficient sample size.