There are people who do many good things, but with pessimism—because to them the world is an inherently bad place. Since their good deeds have no life to them, who knows how long they can keep it up?
We must know that this world is not a dark, sinister jungle, but a garden. And not just any garden, but G‑d’s own pleasure garden, full of beauty, wonderful fruits and fragrances, a place where G‑d desires to be with all His essence.
If the taste to us is bitter, it is only because we must first peel away the outer shell to find the fruit inside.