He desinstalado e instalado Homebrew 3 veces porque parece que nunca me permite instalar nada, ya que me niega los permisos al final de la mayoría de las instalaciones.

Como ejemplo, publicaré este escenario de descarga de libjpeg que estoy enfrentando actualmente.

Intento instalar libjpeg y obtengo:

$ brew install libjpeg
==> Downloading https://downloads.sf.net/project/machomebrew/Bottles/jpeg-8d.mountain_lion.bottle.1.tar.gz
Already downloaded: /Library/Caches/Homebrew/jpeg-8d.mountain_lion.bottle.1.tar.gz
==> Pouring jpeg-8d.mountain_lion.bottle.1.tar.gz
Warning: Could not link jpeg. Unlinking...
Error: The brew link step did not complete successfully
The formula built, but is not symlinked into /usr/local
You can try again using `brew link jpeg'
Error: Permission denied - /usr/local/opt/jpeg

'brew link jpeg' resulta en

Error: Permission denied - /usr/local/opt/jpeg

Esto es lo que lee mi doctor de cerveza

$ brew doctor
Warning: "config" scripts exist outside your system or Homebrew directories.
./configure scripts often look for *-config scripts to determine if
software packages are installed, and what additional flags to use when
compiling and linking.

Having additional scripts in your path can confuse software installed via
Homebrew if the config script overrides a system or Homebrew provided
script of the same name. We found the following "config" scripts:

/Library/Frameworks/Python.framework/Versions/2.7/bin/python-config
/Library/Frameworks/Python.framework/Versions/2.7/bin/python2-config
/Library/Frameworks/Python.framework/Versions/2.7/bin/python2.7-config
Warning: You have unlinked kegs in your Cellar
Leaving kegs unlinked can lead to build-trouble and cause brews that depend on
those kegs to fail to run properly once built. Run brew link on these:

jpeg

Este problema de permisos ha hecho que sea imposible usar brew en cualquier cosa y realmente agradecería cualquier sugerencia.