Read the following text and answer the questions below:
The Legend of Malin Kundang
Once upon a time, in a small village on the coast of Sumatra, lived a poor woman and her son, Malin Kundang. Malin's father had passed away when he was a baby, and his mother had to work hard to support him. Malin was an intelligent and diligent boy. When he grew up, he decided to go to the city to seek a better life.
Years passed, and Malin became a successful merchant. He married a beautiful woman and lived in wealth. However, he forgot about his mother, who was still living in poverty. One day, Malin's ship docked near his village. His mother came to see him, but Malin refused to acknowledge her. He was embarrassed by his mother's shabby appearance.
Heartbroken, his mother prayed to God to punish Malin for his arrogance. Suddenly, a storm came, and Malin's ship was wrecked. He turned into stone as a punishment for his ungratefulness.
What is the main theme of the story?