For Blake, the stars symbolize the power of cold reason, which he greatly despised. Like many of his contemporaries, Blake was deeply impressed by the rapid advancement of scientific knowledge. But, at the same time, he strongly opposed what we would now call scientism, the idea that everything in life can be explained by science. Blake believed that there was a whole world of imagination for which science could provide no adequate account, and he explored that world in considerable depth both in his poems and his numerous artworks.
Although the tiger is an animal and therefore part of the natural world, there is something about it that can't be captured by scientific analysis. Science can tell us so many things about tigers, as it can with all other creatures upon this earth, including ourselves. But what it can't do is convey the sheer awe and terror that the tiger inspires; only the aesthetic imagination can do that. The sense of awe and fear which the speaker feels in the presence of the tiger defies reason. In response to the tiger, reason—as symbolized by the stars—can say nothing. All it can do is suspend its hostility to the imagination ("When the stars threw down their spears") and weep in the presence of this sublime, awesome creature ("And water'd heaven with their tears").