The phrase "get a life!" ultimately means "get a life like mine", and expresses contempt for something that is very important to someone. It is usually said when people tell someone about their interests, which the listener finds unworthy in some way - uncool or boring or too serious or not serious enough.
Most people think the way they are living is the best way to live, and that more people should be like them. But to imply that someone else's way of life is unworthy, and to negate or disrespect their interests, shows a high level of intolerance of difference.
The differences between people, both individual differences and the differences between groups and cultures, are what make life interesting (not always easy, but interesting). Tolerant people acknowledge that there are many ways to live a worthy human life. They can accept and respect different lifestyles, interests and behaviours.
People all have their chosen lives, and while some may seem less worthy, less healthy, less interesting or more trivial than yours, your reaction to them says more about you than about them. Being capable of accepting difference means you are stronger, more gracious and more certain of yourself than accusing others of not having a life.