When It's Better to Leave a Relationship?

How do you know when it's better to leave a relationship and deal with the loneliness of being alone? The key questions are these, can i live the rest of my life in this relationship as it is to day and experience a wonderful life. Is that possible in my current relationship, if nothing changed from the way it was to day? And if answer is no, the question to ask is so therefore it needs to change or i need to leave. What evidence do i have that this is going to change?"