The guy that wrote the GPL licence wrote it so as to protect the rights of the software's end-user so that he/she could be free to know what the software was doing and would have the liberty to change anything they wanted. The problem is, this takes away the privacy from the person who made the software. If he found a piece of code he needs and it happens to be GPL, he is forced to opensource his entire program. I have nothing against open source but I do like the ability not to use it if I ever come up with something I don't feel like giving away.
An example of this was the OGRE engine, which until a while ago was GPL. Anything you made with it had to be GPL also. They've changed this to LGPL so the engine can now be used by more people.
The Irrlicht engine uses the zlib licence, which basicaly says: do what you want with this, but give credit where credit is due. That I like.
That's my main problem with the GPL. There are other little annoying things like the big headers on each code file, and the fact that the executable has to be distributed with a copy of the GPL.