To understand your own language and culture better (from Evona York)
Sometimes learning a foreign language helps you understand your own language and culture better through comparison, or through the relationship between the foreign language and your mother tongue. For instance, studying Latin in high school taught me an incredible amount of English, because English has so many words that come from the Latin. Same with Greek. Of course learning Italian and Spanish give us the same benefits as learning Latin except it's better because they are actual languages in use.

Wer fremde Sprachen nicht kennt, weiß nichts von seiner eigenen
Those who know no foreign language knows nothing of their mother tongue.
(Johann Wolfgang von Goethe)
To keep your mind healthy (from Harvey Schmidt)
Learning a second language has been proven to delay the onset of dementia.

To find your future husband/wife (from Rico Suave)
Learning a new language and culture increases the size of your selection pool.

To better understand the rest of humankind (from Philip Lightfoot)
Language is an aspect of humanity, and learning what a fellow human speaks teaches you more about humankind as a whole. The more languages you know, the more you understand our species, and that is beneficial no matter what the situation is.

To talk to friends without others understanding (from Elizabeth)
If you and some of your friends learn a foreign language, you'll be able to talk to each other without other people having a clue what you're saying. This works best if you choose a language few people study, such as Japanese.

To learn songs in other languages (from Michelle Kelleher Tietz)
Michelle has been learning Irish and Scottish Gaelic because she likes to sing and has founded an a cappella group that sings in Celtic languages. They sing old work songs, lullabies, etc. This hobby combines her interests in languages, history and story telling.

To help people in need (from Nuntawun Yuntadilok)
Learning other languages increases our chances of providing help to people who seriously need it, such as those in hit by the tsunami of 2004, many of whom can not communicate in English, especially children.

Friendship (from Bill Conwell)
My best friend is French and speaks English, Spanish and some German as well. I am trying to learn French to enhance our communication, though he doesn't demand nor expect it.

Venting your feelings (from Dick H.)
Another reason why one might want to learn a foreign language is to insult/cuss out people without them understanding what you're saying. It works best if you choose a language few people study. Avoid the Big Two (Spanish and French), because too many of us learned them in high school. And you never know who has a German, Finnish, Italian, or Greek grandmother!

To help you understand how other people think (from Orionas)
Language influences culture, so learning a language helps you to understand how other people think, and it also helps you to get a general understanding of our world and the many people and cultures that inhabit it.

Because I love learning languages (from Cody Warren)
My reasons for learning foreign languages is because I just love to do it, it's all the different ways people express themself, and it shows that there really are other people out there (to a teenager who's always stuck at home in a small farm town). Its also a way to learn how people interact and socialize with each other, and how each society works.

Also, I like conlanging, and learning different languages help me to understand how things work so i can build conlangs, and vice versa, creating the conlangs help me to understand how all different languages grammars work (Im always a sucker for using a lot of moods, cases, tenses, etc).

Learning languages to me, is much more than making myself able to communicate with others. To me it's like getting some nice new surprise and a whole new level of understanding.