The Four Thoughts that Turn the Mind to Dharma are:

impermanence, suffering, death and karma.

His Holiness the Dalai Lama said in The Joy of Living and Dying in Peace :

It is helpful to learn about the advantages of doing death meditation and
the disadvantages of ignoring it. 

First, it is meditation on impermanence and death that inspires you to engage in spiritual practices. It is an eye-opener. When you become aware that sooner or later you have to leave this world, you are bound to be concerned about the affairs of the next life. This awareness automatically helps you to turn to spiritual pursuits.

Second, death meditation is a powerful technique that helps you to prolong and continue your spiritual practice.  In any endeavor of substance, be it spiritual or temporal, difficulties and problems are bound to occur.  The power of death meditation helps you to face whatever hardships may come your way. 

Finally, this meditation acts as a stimulus, helping you to successfully complete your practice.  Therefore, awareness of death is essential at every stage of your spiritual life.  As a practitioner, you will be more concerned about the affairs of the life after death.  And by eliminating deluded thoughts and actions, you will be able to make this life meaningful.

 There are many disadvantages in not remembering death.  

When you forget death, there is very little chance of your being inclined toward practice.  Without awareness of death, your practice will become slack and ineffective.  You will be predominantly occupied with the affairs of this life.  There are people who receive vows and recite their prayers daily. But because their awareness of death is weak, they behave like ordinary people in times of crisis, becoming excessively angry, attached, or jealous. 

There is a saying  in Tibetan: "When you are well fed and enjoying the sunshine, you look like a practitioner.  But when faced with a crisis you reveal your true nature."  Everyday experience tells us that most of us are like this.  

Without awareness of death, you have the affairs of this life at the center of your heart. And because you are obsessed with wealth, status, and fame, you barely flinch when committing negative actions. 

A person who is not concerned about death naturally has no interest in the lives beyond it.  Such an individual has no great regard for spiritual values and readily cultivates deluded thoughts and actions.  Consequently, such a person is a source of harm to himself and others.

If you forget that you will die, you will think mainly about how to lead a prosperous life. Your most important concern will be to get a good place to stay, good clothes to wear, and good food to eat. You will not hesitate to deceive and threaten others if you get the chance.  What is more, you might judge such negative activities as the marks of an efficient and capable person.  This is a clear indication that you are not farsighted enough to think about the long future ahead. 

We all have many lives to come, which are completely dark to us and about which we have no idea.  When you forget these circumstances, you will be inclined to pursue destructive activities.

~ extract of HH Dalai Lama's The Joy of Living and Dying in Peace  courtesy E. Judd at