Housman compares life to the blossoming of a cherry tree because a person's life is much like a cherry tree.
A cherry tree blossoms like a young person. However, the blossoms do not last forever, much like life. Therefore, we have to enjoy the blossoms -- and our lives -- while we can.
He says that if you only enjoy the cherry tree when it blossoms (in the spring) then you will not get a lot of time to enjoy it. That's why at the end of the poem he goes "to see the cherry hung with snow." He will enjoy the cherry tree all year round, instead of just once a year.
In the same way, we have to enjoy life all the time, not just some of the time.
