I'm just curious - is the U.S-Japan alliance maintained more by mutual desire and consent or mutual necessity?

Question:

Answer:

I believe Japan-US alliance will last for at least another few decades because of "mutual necessity."

I want to emphasize the mutual defense alliance, which is crucially important for both countries.
By maintaining US Forces in Japan, the US can promptly dispatch their military, including USN 7th Fleet to all over Asia-Pacific region in times of contingency. We know they are also deployed to the Middle East which is outside the framework of Japan-US Defense Treaty. But, I am not going into this issue here.

Japan, on the other hand, has poor defense capability due to its Constitutional limitations (which was written by the US right after WWII when Japan was occupied by the Allied Powers). Japan needs USF presence in Japan whether we like it or not. Therefore, Japanese tax payers are paying enormous amount of money to assist US Forces in Japan every year.

So, this is a mutual alliance for mutual national interests.