Angus Tuck tells Winnie that the life cycle is like a wheel, ever turning, and that life is like the river, the water always moving. When one thing does not die, even something as small as a mosquito, it is not following the natural order of things. The Tucks are stuck, like a rock, at the side of the river, just watching everything else go by, living and dying, and they never change or grow old. If all mosquitos lived forever, they would prey on animals and humans, and if they never died, then that means that animals that rely on mosquitos for food would not be able to eat them (hmm, how would that work exactly?). And if mosquitos kept breeding, they would quickly overrun the world, and there would be no way of controlling mosquito populations. What a disaster that would be!
If mosquitoes did not die, the food chain would be drastically altered. Species that are dependent upon mosquitoes for food would obviously either die out or begin to feed on other species whose populations would be affected by such changes. For animals that are fed on by mosquitoes, there would be no relief from them. Mosquitoe-born disease would run rampant, and victims would die in mass numbers. Essentially, the whole food chain would be thrown in a state of disarray. Death is important because it makes way for new life. Without death, there is no balance for the living.