It's either he just exists, but there is no reason.
Or he exists, and there is a reason. And if there is a reason, I would say it's because he must exist.
I would say no reason at all is irrational. Therefore, the reason is that he must exist.
Or he exists, and there is a reason. And if there is a reason, I would say it's because he must exist.
I would say no reason at all is irrational. Therefore, the reason is that he must exist.