spoilers
I don't know if I can describe it accurately but from what I remembered from the novel, the spore is like his own kind, you can say it's his offspring or child or something since it came into existence because of him (not sure if it came out from him, but you get it)
The spore matters to him greatly because it's his life's purpose. To live its short life and give birth or life to another baby mushroom, take care of it and pass away.
You can stop here if that pretty much satisfies you but scroll down if you want to be spoiled more XD
Later at the resolution, that spore is a catalyst for our MC to once again have second chance in life, I won't elaborate on this. But basically, the spore has his perfect replica or DNA whatsoever, so with science and time, he was back:3
Is the spore like his soul?