The speaker decides to go to the woodlands and see the cherry tree because he knows that life is short, and that he might not have many more chances to look at nature. He tells us that he will most likely live "threescore years and ten," which means 70 (3*20+10). This is considered the normal span of human life according to the Bible. As the speaker is twenty, he has fifty springs left, which he argues is not enough time to truly enjoy everything that he still has to see.
The speaker can be best described as someone who appreciates nature and the simple things in life. The fact that the speaker thinks so carefully about the length of his life demonstrates a lot of maturity. Moreover, the speaker clearly values nature, as this is the main thing he wants to enjoy before he passes away.
