People live in the past because that's where their thoughts are. Whether those thoughts are focused to one area in particular or a whole range of reasoning, it is an issue of self evaluation or comfortable living. Because it is the past, people are more comfortable with the certainty of it, even if it was a negative experience. Like common human thought-stops, there is a fear of the unknown, and the future is full of it.
While it is true that those "who know not of the past are doomed to repeat it" it shouldn't apply to the short confines of your life unless you are a truly forgetful person. To know the past isn't always the same as it is to reflect upon and understand it. To be aware of a past is almost pointless unless you carry the knowledge of its meaning.
For those with the knowledge and understanding of their pasts then it is only logical to move forward, and often better health-wise if the matter of the past is an upsetting one. Times change, people grow older and there is always another reason for letting go. The heart will get better and tears will cease with age. Pride will be repaired and mistakes will be forgiven, but only if there is a desire to change. Being stubborn and set in past ways will only make the transition to the next step that much more difficult.
Unfortunately advice often happens to be much easier said than done and many people are unable to remove themselves in fear of the outcome. If an outsider tries to intervene without understanding the reason for the obsession then nothing will happen or deeper scars will be left behind. Change and acceptance must be persuaded so the impact occurs on a slower and more gradual scale. It is not good to shock the system to force acceptance to fact. And luckily this gradual acceptance isn't geared towards one situation either, but many scenarios could apply from heartbreak, to death, to separation, job termination, and many more.
The past is important to everyone and vital to understanding. If it wasn't for a foundation set in the past, lives built in the future would only be doomed to collapse. This is why it is good to embrace the past, but not live by it. For if one was to only live at the foundation of a structure, would they ever see the sun and the sky? Or would we be looking solely at the dirt and questioning out time left to us before returning to it. Come out of the cellar and step towards the outside, look up and see the sky, just before building your way towards it.