Purgatory

Someone please explain the definition of lonely.
Is it the same as being in purgatory by your self, but then again doesn't that bring you back to lonely? Or is it the absence of another body or being? Is it the same feeling when you go off to kindergarden and you turn around for one second and when you turn back your mom is gone?
Can someone explain lonely to me an in the process explain the reason for purgatory because from what I know, why have purgatory when you can just be lonely forever?

I know this is hardly making sence to alot of people but its the only way I know how to get this off my chest.

Purgatory=LONELY