The character knows that once it leaves the world he was born, he expects to be unable to commune with it. Any power he has will be granted by the seed. It's like a symbiotic relationship. The seed grants the character power, and the character will seek out the best place for the seed to grow. The world tree's motivation is it has lived for so long, it is seeking to procreate onto another world. Whatever world that happens to be is beyond the tree's control which is why it was granted to a seed bearer to seek the best place of that world.