Lightdm is a nice display manager, which best I can tell does nothing other than provide a nice graphical login screen (someone correct me if I’m wrong). But I sort of built up my install from bits, and I hadn’t been able to get lightdm to work, so I was always starting xfce via startxfce4, and logging out via xfce4-session-logout (there might have been a better way to do that latter bit). But I decided to give it another go, and found that you can simulate lightdm via “lightdm –test-mode”, which told me that lightdm was starting then immediately crashing. Not terribly helpful. But wait there’s more, lightdm also accepts a debug flag, so “lightdm –debug –test-mode” told me something useful.
[+0.13s] DEBUG: Failed to load session file /usr/share/xgreeters/default.desktop: No such file or directory
Interesting at least, so I browsed over to that directory, and found that unity-greeter.desktop was the only thing in there. So I just symlinked from the only option to where lightdm seemed to be looking and voila, it started. You might have a different issue, but hopefully getting some good debug output will help.