1. He was simply curious, his dad and uncle were both merchants, and ever since when he was very little he would go to different places with his dad and uncle and explore. And when he heard that he had the opportunity to go to China, he seized upon the the opportunity and went.

2. Possible profits: Because he was from the merchant class he thought that he could get a drastic gain on trades and potentially a lot of money if he had gone to China and opened up trading, that was also why he went to China.

QUOTE from http://www.thenagain.info/webchron/china/MarcoPolo.html

1275-1292
Marco Polo was born in Venice, Italy in the year 1254. He had an education of different skills in accounting, foreign languages, and knowledge of the Christian Church. His background in business and culture and his love for nature made Marco Polo very observant of humans, animals, and plants.
His father, Nicolo, and his uncle, Maffeo, were merchants who began their first eastern journey in 1260. They visited Constantinople and made their way to the domain of the Great Kublai Khan, ruler of China. The Emperor became interested in stories of the native land of the merchants; thus, he sent the Polos back to the Pope as his ambassadors with messages of peace and interest in converting areas of China to Christianity.
The merchants remained in Venice for two years and decided to keep their promise of return to Kublai Khan. Large profits from trade with these distant parts also prompted the brothers to return. On this journey, they took the seventeen year old Marco Polo with them. After three and a half years of travel, the ambassadors humbly appeared before the Emperor.
China had matured in the arts, both fine and practical, beyond anything found in Europe. Literature was greatly respected. Paper had already been invented; books of philosophy, religion, and politics could be found and a large Encyclopedia had been printed under the supervision of the Emperor. Mechanical devices were not lacking and paper money was the accepted currency in many sections of the empire. It was in this world of advanced wonders that Marco Polo resided for many years.
Upon his return to Italy, Marco Polo told of his findings of jade, porcelain, silk, ivory, and other riches of Asia. He described the festival of the Emperor's birthday in which everything from clothing to ornaments were laced in gold. He also explained how he saw people using black stones for fuel (later known as coal). Unfortunately, all his stories and details of the unimaginable were rejected, and Marco Polo became the "man of a million lies."
After he retrieved his notes from China, Marco Polo transformed his travels into manuscript form. His work has been criticized because he did not include fundamentals of Chinese life as tea, foot-binding, or even the Great Wall. He was frank, unpoetic in imagination and vision, and constantly spoke of trade, money, risks, and profits (as an ordinary business man/merchant would do). However, he wrote in incredible detail of the birds animals, plants, and other aspects of nature.
When he was near death, a priest entered his room and asked him if he wanted to admit his stories were false. Instead, Marco Polo replied, "I do not tell half of what I saw because no one would have believed me."

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