failsafe x: low graphics mode

[warmonkey]

solution tried:
sudo apt-get install --reinstall xserver-xorg lightdm
sudo dpkg-reconfigure xserver-xorg lightdm
sudo Xorg -reconfigure

finally i checked log file /var/log/lightdm/lightdm.log
it shows that lightdm cannot open /var/lib/lightdm/.Xauthority
my solution:
 mkdir /var/lib/lightdm
    chown lightdm:lightdm /var/lib/lightdm
and it works

so, why reinstalling lightdm can't create /var/lib/lightdm