William Blake was a Romantic poet, and as such, he often in his poetry celebrated the natural world. Romantic poets celebrated the natural world as a refuge from the hustle and bustle of the city, and they also believed that by being one with nature, one could be close to God, who was manifest in nature. Romantics also believed that the value of an individual's life could best be determined by that individual's emotional wellbeing.
At the same time that Blake was writing, England was undergoing an industrial revolution. The industrial revolution was a boon for capitalism because it allowed property owners to use machinery rather than people and thus maximize their profits. This prevailing capitalist ideology therefore prioritized personal gain and encouraged the belief that one's worth, or value, could be determined not by one's emotional wellbeing, but by one's social and economic statuses.
The industrial revolution also posed a threat to the natural world. More and more factories were built, which meant more and more pollution. The natural world was considered important only in as much as it provided the natural resources to power industry.
From a modern perspective, William Blake's mindset, and that of the Romantics generally, seems very prescient. We appreciate the natural world much more now because we understand global warming and the consequences of industrialization.
From William Blake's Romantic mindset, we might still gain a better understanding as to how to live in harmony with the natural world. We might also gain an understanding as to the consequences of thinking of the natural world as a resource to exploit rather than as something to appreciate and cherish.
From the same mindset, we might lose in that our industrial and technological progress would be necessarily slower. If people had appreciated the natural world more in Blake's time, then it is possible that many people in the developed world today wouldn't have quite the same array of luxuries that they take for granted and which are consequent of rapid industrial progress.