Learning new languages makes you smarter. Obviously, because you know another language, but it also engages your brain with something new, different. It'll make you more capable of figuring out more challenging problems.
Also, if you learn a new language, you can go to more places and be able to actually SURVIVE there. For example, if you went to, say, France, but you don't know how to speak French, it would be a little difficult to get around, right?
New languages can also be fun to learn as you connect your native tongue to the new language and realize how many similarities there are, whether those similarities are in reading, writing, grammar, or pronunciation. I don't know about you, but it kind of fascinates me when I find, like, a root word and connect it to all of the words with that root word. Or maybe I'm just weird.
So if you are deciding whether or not you should learn another language, well, I say, go for it!