The story of Life cycle of Birth and Death. Why it happens?

The following story will help us understand how the living entity changes from one body to another based on his desires. Once there lived a person named Kailash who was very attached to his house. One day the great sage Narada came to him and said, "Kailash, come with me. I am going to Vaikuntha (the kingdom of God). I will take you along with me." Kailash replied, "Sorry sir, my children are very young. I have to take care of them. Maybe later."

Several years later Narada returned to him and asked, "Kailash, ready now?"

Kailash replied, "Sorry sir, now I have my grandson to take care. Also everyone at home needs my guidance. I cannot come now."

When Narada returned after many years, he came to know that Kailash had passed away. While leaving he noticed a dog barking loudly at the doorway. The dog said, "O Naradaji, I am Kailash here" Surprised Narada replied, "O Kailash, you have become a dog now! What are you doing here? Even now, it is not too late. I can take you to the kingdom of God. Would you like to accompany me?" The dog replied, "Sorry sir, my sons are very careless with the wealth I acquired. I have therefore become a dog to protect that wealth from thieves." Feeling very sorry for the attached Kailash, Narada went away. After many years when Narada returned, he learnt that the dog had died. Thinking sadly that Kailash must have gone to some unknown body, Narada departed. When he was walking behind the house in the paddy field, a snake came hissing on the way. The snake said, "Narada, I am Kailash here, I am Kailash here." Narada asked him, "What are you doing here in a snake's body, Kailash? Don't you want to come with me now? Even now it is not too late."

The snake replied, "Now I am happy that my sons have put all the money in a bank. But they are so lazy that they wake up very late in the morning. By that time, all the grains in this paddy field are either destroyed by elephants or stolen by neighbors. I have therefore become a snake to go round and round the field to protect the grains. I am very, very busy. I am sony, I cannot come with you now also."Seeing the pitiable condition of Kailash, Narada muni clapped his hands loudly, called the sons of Kailash and showed them the large snake living in their field. Immediately the sons of Kailash started hitting the snake with sticks. Kailash in his snake body tried to tell his sons, "I am your father and I am protecting your paddy fields for your sake. Why are you killing me?" But the more he hissed, the more his sons hit him till he finally died. While dying the snake remembered Narada and in his next life he became a great devotee of Lord Krishna by Narada's grace. Thus a person is impelled to accept one body after another according to his desires and actions. Such a person, being attached to his material plan-making business, spends all his time in bodily consciousness and goes on suffering in the cycle of birth and death.