why do we learn a lagauge?

Learning a foreign language takes time and dedication. The reasons below may help to convince you to take the plunge, if such persuasion is needed. Some reasons are practical, some aspirational, some intellectual and others sentimental, but whatever your reasons, having a clear idea of why you're learning a language can help to motivate you in your studies.
     So, why should I learn a laguage?
        

Emigration

When you move to a different country or region, learning the local language will help you to communicate and integrate with the local community. Even if many of the locals speak your language, for example if your L1 is English and you move to the Netherlands, it's still worth your while learning the local language. Doing so will demonstrate your interest in and commitment to the new country. 

Family and friends

If your partner, in-laws, relatives or friends speak a different language, learning that language will help you to communicate with them. It will also give you a better understanding of their culture and way of thinking.

Culture

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.

Travel

Many English speakers seem to believe that wherever you go on holiday you can get by speaking English, so there's no point in learning any other languages. If people don't understand you all you have to do is speak slowly and turn up the volume. You can more or less get away with this, as long as you stick to popular tourist resorts and hotels where you can usually find someone who speaks English. However, if you want to venture beyond such places, to get to know the locals, to read signs, menus, etc, knowing the local language is necessary.

Study or research

You may find that information about subjects you're interested in is published mainly in a foreign language. Learning that language will give you access to the material and enable you to communicate with fellow students and researchers in the field.

Work

If your work involves regular contact with speakers of foreign languages, being able to talk to them in their own languages will help you to communicate with them. It may also help you to make sales and to negotiate and secure contracts. Knowledge of foreign languages may also increase your chances of finding a new job, getting a promotion or a transfer overseas, or of going on foreign business trips.

Studying abroad

If you plan to study at a foreign university, college or school, you'll need a good knowledge of the local language, unless the course you want to study is taught through the medium of your L1. Your institution will probably provide preparatory courses to improve your language skills and continuing support throughout your main course.
                  these are some of the reasons why you should learn other langages, but there still other, maybe if you ask some one why do learn foreign language he will say" because i love doing this!!!..." it's simple he loves doing this because language is the way that people use to exprece themselves! and  by learning it you can ynderstand your own language, i also believe that when somebody at least tries to learn the language of the country they are visiting it shows a lot of respect. People really appreciate it. in the end you'll have much fun by learning it!!!!....