Some people do many good things, but with pessimism, because for them the world is an inherently bad place. Since their good deeds have no lasting impact on them, who knows how long they will continue doing them?
We must understand that this world is not a dark and sinister jungle, but a garden. And not just any garden, but God's private garden, full of beauty, wonderful fruits and fragrances, a place where God desires to be with His very essence.
If the taste is bitter to us, it is only because we must first peel the outer shell to find the fruit inside.