The self defense nature of their military was written into their constitution by the US. They have been gradually making changes that allow more and more involvement outside of Japan but the culture still persists. The primary reason they (Japan) are involved and supportive (other than being a western democracy) is that Russia is one of their oldest adversaries and they have had an ongoing territorial dispute with Russia for decades. Supporting Ukraine is a cheap way to weaken your adversaries. Also, it is a stretch but not inconceivable that Japan’s territorial disputes (and others) could be made part of a future peace negotiation with Russia at the end of this.