You knew this one was going to pop up eventually.
The US doctrine of fair use has been around pretty much as long as the copyright law itself, though it wasn't codified until 1976. There is also the related doctrine of fair dealing found in many other countries around the world. For our purpose here in examining our third Happy Thought, we can combine the two. (For a summary of the history of fair use in the United States and internationally, see Wikipedia.)
Fair use and fair dealing both serve to make copyrighted works (and in a way, copyright law) more accessible to the people. These exceptions to copyright protection provide a mechanism that allows people to use works in certain situations without having to deal with the complications or expense of licenses.
Legislatures, courts and other law-making bodies chose the types of uses that fall under these exceptions based on a variety of determinations. There is the social benefit aspect to fair use and fair dealing where the encouragement of creative growth and development is desired. There is the economic aspect that tries to solve the problem of logistically cumbersome licensing requirements for small users. And there are various other factors like history and culture, etc.
These are all part of the reason fair use and fair dealing are good aspects of copyright. Like the US Homestyle Exemption of the last Happy Thought, these exceptions are about creating a balance between copyright users and creators that benefits society but still considers the copyright owner.