Why does this happen?

pip install -E ../../conf/requirements.txt 
Traceback (most recent call last):
  File "/home/alecs/workspace/Homepage/env/bin/pip", line 5, in <module>
    pkg_resources.run_script('pip==0.7.1', 'pip')
  File "/home/alecs/workspace/Homepage/env/lib/python2.6/site-packages/setuptools-0.6c11-py2.6.egg/pkg_resources.py", line 489, in run_script
  File "/home/alecs/workspace/Homepage/env/lib/python2.6/site-packages/setuptools-0.6c11-py2.6.egg/pkg_resources.py", line 1207, in run_script
  File "/home/alecs/workspace/Homepage/env/lib/python2.6/site-packages/pip-0.7.1-py2.6.egg/EGG-INFO/scripts/pip", line 3, in <module>
    sys.exit(pip.main())
  File "/home/alecs/workspace/Homepage/env/lib/python2.6/site-packages/pip-0.7.1-py2.6.egg/pip/__init__.py", line 94, in main
    return command.main(initial_args, args[1:], options)
  File "/home/alecs/workspace/Homepage/env/lib/python2.6/site-packages/pip-0.7.1-py2.6.egg/pip/basecommand.py", line 103, in main
    complete_args)
  File "/home/alecs/workspace/Homepage/env/lib/python2.6/site-packages/pip-0.7.1-py2.6.egg/pip/venv.py", line 50, in restart_in_venv
    [python, file] + args + [base, '___VENV_RESTART___'])
  File "/usr/lib/python2.6/subprocess.py", line 633, in __init__
    errread, errwrite)
  File "/usr/lib/python2.6/subprocess.py", line 1139, in _execute_child
    raise child_exception
OSError: [Errno 13] Permission denied

Thanks.