There was a good doctor.  He was wise and intelligent.  His medical skill was superior, he can cure various diseases.  And he had many sons.

One day, the doctor went away from home to visit his patients in other country.  After the doctor left, his sons drank the poison by accident.  When the poison in their body went into action, they cannot take the pain and suffering but rolling on the floor.  Some of the sons were heavily poisoned and lost their perceptions, but rest of them did not.  In the mean time, their father came back home.  All of them were very happy to see their father coming back, and they all kneeled down and folded their hands to welcome their father coming back home safely.  The sons told their father that, they were stupid and they drank the poison by accident.  They asked their father to relieve and treat them.  The father saw his sons suffering from the poison, he wrote the prescriptions for his sons, and bought the best herbs for them to take.  For those sons who were not heavily poisoned, they took the best medicine right away and get cured.  For the sons who were heavily poisoned, although they were folded their hands to welcome their father coming back home and asked their father to treat their illnesses.  But they did not want to take the best medicine that their father gave them.  They said this medicine was not the best medicine.  The father thought, these sons were very pity, they did not want to take the medicine because their perceptions were upside down.  Although they were very happy to see him, and asked him for the best medicine to neutralize the poison.   He should think of a good method to make them taking the medicine.  The father immediately told those sons that, they should know he was senile and could be die at any time.  He left the best medicine here so that they can take it.  After the father saying that, he went back to other country to visit his patients again. 

After this good doctor arrived the destination, he sent a messenger back to tell his sons that, your father is dead.  The sons were very sad and worry when they heard the bad news.  They thought, if their father were still alive, he would compassionately relieve and nurse them whenever they were sick.  Today, their father renounced them and died in other country, they became orphans and they did not have father to depend on.  The sons were sadly remember their father with longing and grateful to him very often.  Because of this, their perceptions were awaking gradually.  Then they realized that the medicine that their father gave them were the best medicine with the best quality.  So they took the medicine immediately, and their illnesses were cured.  As soon as their father heard that all of his sons were cured, he came back home to see them.

Similarly, Buddha is like this good doctor, he went through countless four asankhya kalpas in the rise, duration, and end of every universe to teach innumerable living beings to enter the Buddha path.  For delivering the living beings, Buddha used an expedient method, that is, entering the Nirvana.  If the Buddha dwelled in the world for long, the sentient beings with less merit would not cultivate the good roots.  They would be in poverty, attach to the five desires arising from the objects of the five senses, and trap in the net of wondering thoughts and false views.  If Tathagata was always abiding and not dying, the living beings would be arrogance and pride, weary and lazy.  They would not generate the thought that 'It is hard to have Buddha dwelling in the world' and the reverence mind.  That's why Buddha used this expedient method entering the Nirvana for delivering the living beings.

As soon as the Buddha entering the Nirvana, the living beings made offerings of Buddha's Sari, they were all remember the Buddha with longing and grateful to him.   Because of this, the living beings generated the reverence mind.  They had cultivated the good roots after they believed in and submitted themselves to the Buddha.  They entered the supreme way or truth that of Buddha, and attained the three fold embodiment of Buddha quickly.