Because you're making the language the end, and not the means. Learning a language isn't the ultimate goal...the ultimate goal is experience, travel, new friends, love, culture, music, food. Find a girl who only speaks the language you're learning and suddenly it won't feel so boring. Otherwise, just learning a language is super boring...classes, books, exercises.

I think you mean "Why do I get bored when *I* learn a language?" Frankly, I don't think *you* care what *I* am learning.