Why was it that Shakyamuni Buddha left home to cultivate the Way? It was because he came to realize that birth, old age, sickness, and death are not easy to endure.

When Prince Siddatha was nineteen years old, he wanted to see the sights outside the palace. So one day he went to the city's east gate. There, he saw a woman in the process of delivering a baby. The Prince asked his followers, "What's happening?" They replied, "She is giving birth to a child." Looking at the woman who seemed to be enduring an extremely painful event and at the newborn child crying loudly, the Prince returned home unhappily.

The next day, the Prince went to the city's southern gate. There he saw a very old man whose hair was totally white and whose eyes were dim. His legs were too weak for him to stand straight or to walk. The Prince asked in surprise, "What's wrong with that man? Why has he become that way?" His followers answered, "This man is already old. He's lived too many years, and so he's that way." Upon hearing that, the Prince became upset and quickly returned to his palace.

On the third day, the Prince went to the city's west gate. There, he saw a sick person. Being sad, the Prince returned to the palace.

On the fourth day, the Prince went to the city's north gate. There lay a dead person. "What's the matter with that person?" asked the Prince. The followers said, "This man has already died." The Prince again felt extremely depressed.

The Prince himself had witnessed the suffering of birth, sickness, old age, and death, and he realized that was the process human beings have to pass through. He felt very sad and wanted to go back to the palace. Right at that moment, a left-home person (that is, a Buddhist monk) appeared. The Prince asked his followers, "Who is this person? What's he doing?" They then went to talk to the cultivator. He told them, "I'm a left-home person. I investigate and study the Buddha Way in order to be liberated from the suffering of birth, sickness, old age, and death."

At that time, the Prince had not yet became a Buddha. But, when he heard that by cultivating the Way he could avoid birth, old age, sickness, and death, he said, "Can I cultivate in the same way that you do?"

The monk replied, "Anybody can."

Then the Prince returned to the palace, and accompanied by one who was later known as Venerable Upali, he ran away.

Shakyamuni Buddha was totally disillusioned by birth, old age, sickness, and death. He didn't know how they arose or where they went to. And so he left the home-life and practiced the Way with the intent to end birth and death. He went into the mountains and cultivated for six years trying to avoid birth, old age, sickness, and death.

The following section is from the commentary on Lotus 15:

The Buddha was about thirty when he realized enlightenment. He was staying in the Himalayas cultivating ascetic practices and eating only one grain of rice and one sesame seed a day. Five of his relatives were staying with him and also cultivating. But three of them couldn't stand the suffering. "We eat so little every day. How can we go on living?" And they ran away. Three left, and two remained. These two felt that cultivation must entail suffering; there should be no enjoyment whatsoever. Feeling that suffering was mandatory, they did not fear the bitterness of cultivation.

Then a heavenly maiden came to offer a bowl of rice gruel with milk in it to the ascetic Shakyamuni, who was by that time nothing but skin and bones; he had no evident flesh or blood. Wouldn't such a person be pathetic-looking? The heavenly maiden brought him some milk gruel. We know that it's all right for Buddhist disciples to drink milk because our founding father drank milk at that time. The Buddha accepted the offering and drank it. When the remaining two cultivators saw that, they gave him a look and said, "He's not cultivating anymore. He's eating such a good thing. It will prevent him from being able to cultivate. Let's leave." And the two of them left as well.

The five relatives and fellow cultivators of the Buddha eventually all left, until only Shakyamuni Buddha himself remained. Then he thought, "I'm leaving too." He wandered for five years and then returned to the Bodhi tree, saying, "I'm not going anywhere else. I've seen it all, I'm going to sit under this Bodhi tree and accomplish the Way." He sat there and thought, "If I don't get enlightened, I'm not getting up from under this tree. He sat for forty-nine days and then became enlightened and accomplished Buddhahood. Not long after he became a Buddha, he took a look at causes and conditions to see whom he should save first. He saw that he should go cross over the five relatives who had been cultivating with him. He looked to see where they were, found that they had gone to the Deer Wilds Park, and thereupon went there to speak Dharma for the five, who became Bhikshus. These five perceived that Shakyamuni Buddha had become a Buddha, and they immediately became enlightened themselves upon hearing him speak the Dharma. The first to become enlightened was Ajnatakaundinya.