What motivates free software licensing decisions?

Free software licenses can be divided into two broad categories: copyleft licenses (like the GPL), which require derivatives of the software to be licensed under the same terms; and permissive licenses (like the MIT/X11 license), which allow the software to be reused in any project, even closed-source projects. There are variations, of course—the LGPL, for example, is a ‘weak copyleft’, allowing licensed works to be used in closed-source works, but requiring improvements to the work itself to be released under a copyleft license.