There are many excellent reasons to learn any language. I think most people learn another language because they need or want to communicate with friends, family members or business partners from that country. It is also important if the learner wishes to study and better understand the culture (arts, music, cuisine, etc.) of that country. In the case of US-Japan there are certainly close business ties, tourism, and there are perhaps many Americans interested in Japanese culture (martial arts like karate, judo, aikido; anime and manga; sushi; etc.) that might motivate a passion for learning the language. I am taking your question to be about native "American English" speakers learning Japanese--as opposed to "native Americans" learning Japanese. A "native American" is someone whose ancestors were indigenous to and lived in America prior to its discovery and colonization by Europeans in the 15th-19th centuries. This is a much smaller population that native "American English" speakers.