There are many reasons!
First, Chinese is the language of Nearly 1/4 of the worlds population. Speakers of Chinese not only live in China and Singapore, but also spread throughout Southeast Asia, North America, and Europe, where large Chinese communities congregate[聚集]. Chinese people today have been playing increasingly important roles worldwide.
Second, learning Chinese is an extraordinary and agreeable[令人愉快的] experience through which you can immerse yourself in a different culture. Unlike most languages, Chinese has a unique ideographic[表意文字的] writing system, which provides visual comprehensibility. The grammatical structure of Chinese is not only logical, but also pragmatic[实用的,实际的], related to the particular way of Chinese thinking. Knowledge of the written language opens up the culture of one of the worlds oldest civilizations.
Third, traditional Chinese culture, from Confucianism[儒家思想] and Chan Buddhism[禅思想] to martial[军事的] arts and Chinese cuisine[烹饪], has an enormous influence on East and Southeast Asian nations. Chinese culture has also greatly inspired the western world through Marco Polo, G. W. Leibniz, Max Weber, Franz Kafka, Andre Malraux, Bertolt Brecht, Ezra Pound, and Luis Borges, among others.
Last, but not least, the Peoples Republic of China currently boasts the fastest growing economy in the world and is widely regarded as the potentially biggest global market in the 21st century. Proficient speakers of Mandarin Chinese will find jobs in various fields such as business, government, international relations, information technology, tourism, education, translation and much, much more. Of all foreign languages at American universities and colleges, Chinese shows the highest proportional increase in enrollment.
Many people take Chinese language lessons for the purpose of working in China in the future, as now there are many opportunities to do so. Some people learn Chinese for general cultural reasons. A few learn Chinese just for challenge, as they believed (but later proven to be wrong) that it is one of the most difficult languages to learn, because Mandarin Chinese is not as difficult as they thought.