Here I am, a little late, with what I think of the phrase.
It's generally said to someone who seems to be focused too much on one thing, hobby, person, etc.. The person saying "get a life" usually means it with a bit of disdain or possibly concern, and probably feels like the person isn't really living if they're limited to one particular thing and not out experiencing new and different things.
Suffering from depression, it's something I've heard a lot, from people who probably meant well, but it generally does no good.
