1. To find your future girlfriend / boyfriend / partner / husband / wife . Ok, let's get one of the more controversial reasons out of the way! Do people really dedicate the time and effort in learning a foreign language to find an exotic partner? You betchya! Even if it is not the primary reason, for many it is seen as a great bonus! Is there anything wrong with this? No way! Unless you already have a girlfriend / boyfriend / partner / husband / wife...
2. Travel. Learning a language or even just the basics will enrich your travel experiences more than you would believe! Instead of resorting to speaking English louder and louder in the hope that you will be understood, learn at least some basics before you go and and get ready to enjoy the warm and friendly reaction you will receive wherever you go. Locals really do appreciate the effort. At least learn the essentials like 'one beer please' or 'you are very pretty / handsome'.
3. Work / Career. Do something that will differentiate you in the workforce, even if it is not directly related to your work. You never know when an opportunity will present itself for some work related travel and your language skills will have you first in line!
4. Self Growth. We are strong believers+ in life long learning and languages are one of the best choices there is. It will expand your knowledge and understanding of other cultures and peoples and teach you more about yourself in the process. You will even gain a better understanding of English in the process! Learning keeps your mind active and will help you stay alert and 'switched on'. Many of the world's greatest poets, writers and scientists could speak more than one language so you will be in great company.
5. Improve your English. Ok, this sounds strange I agree, but anyone who has studied a foreign language will tell you that they have learned more about their native language during their foreign language studies. Your technical study of a foreign language will also be aplicable to English in many ways and you will probably learn things about English that you never really learnt properly at school and currently take for granted.
What's your best reason for learning another language?