Somebody with no life is one that doesn't value their own time, and so will spend it in ways that even they feel or know are a waste of their time (although they may not admit it).
As different people enjoy different things and priorities vary by person, this is often used (by people who disagree with what is or isn't a good use of time) to judge and insult people who are actually happy with the way they spend their time.
Alternatively, it could refer to somebody who is very dedicated to something (especially something frivolous or negative), at the cost of most anything else.
1: LOL U LIKE THAT? OMG LOSER FAGET
GET A LIFE
1 (translated): Wow, I can't believe you spend all your time doing something you enjoy that I don't, you must have no life.
2: Yet you have nothing better to do than judge people like a prick
. It's ironic that you sound so full of yourself, since it makes it seem like you think making fun of somebody's hobbies means you have a life.
3: Are you two seriously arguing over who does or doesn't have a life? Both of you need a life!
4: Time is one of life's most precious resources. Do not squander it by having no life, or by listening to those that say you have none.
5. Unless you're hurting people with what you're doing. In that case, get a life.