1) To increase your communication skills
Learning other languages will help you communicate with people from all over the world and easily integrate in different communities.  
2) To make lifelong friends
If your friends, relatives or colleagues speak a different language, learning that language will help you strengthen the bond between you. It will also give you a better understanding of their culture and way of thinking.  
3) To improve employment potential
Knowledge of foreign languages will increase your chances of finding a new job, getting a promotion, obtaining a transfer overseas, or going on foreign business trips. If your work involves regular contact with speakers of foreign languages, being able to talk to them in their own languages will help you build that competitive edge. The knowledge of a foreign language puts you at a great advantage.
4) To improve chances of entry into college or graduate school
You may be required to study a particular language at school, college or university. Additionally, languages are a great plus if you are planning on obtaining a T.A. position at college, for example. While doing research, you may also find that information about subjects you're interested in is published mainly in a foreign language. Learning other languages will give you access to other material and versions and enable you to communicate with fellow students and researchers in the field.
5) To make travel more feasible and enjoyable
A basic ability in a foreign language will help you 'get by' while ordering food and drink, and finding your way around. You may also have pleasant conversations with the native people that you meet, adding a new dimension to your holiday.
6) To expand study abroad options
If you plan to study at a foreign university, college or school, you'll need a good knowledge of the local language both for your studies and for your personal life.
7) To increase understanding of oneself and one's own culture
If your heritage language is Spanish, you will surely want to learn it and possibly teach it to your children. It could also be useful if you research your family tree.
8) To appreciate international literature, music, and film
Maybe you're interested in the literature, poetry, films, TV programs, music or some other aspect of the culture of people who speak a particular language and want to learn their language in order to gain a better understanding of their culture. Different languages protect and nourish the growth of different cultures, where different pathways of human knowledge can be discovered. They certainly make life richer for those who know more than one of them.
9) Linguistic interest
Maybe you're interested in linguistic aspects of a particular language and decide to learn it in order to understand them better.
10) One language is never enough!
Whether you are interested in linguistics, enjoy the challenge of learning another language or would like to increase global understanding, we all know that one language is never enough!