I believe the poem is trying to say that even though there is bad in the world, we as humans have a choice as to whether we want to allow that to affect our pursuit of happiness. Appreciate life for what it is, but you have to determine that for yourself.
Basically, the poem can be summed up in the last three lines"
"it is still a beautiful world.
Strive to be happy."
These statements, however simple, are very profound. And frankly, they are principals many individuals forget in a modern world.