Both thoughts and feelings are natural. Good & bad is a statement of the mind and has nothing to do with feelings, yet serves as a medium for the intellectualization of emotions, such as categorically saying, "Feeling bad is natural." Such a medium is the misappropriation of what is natural.
Most people who have a heart cry when they grieve the loss of a loved one. People might say the person feels bad, but they don't in that situation say that feeling bad is itself bad. No one mourns who does not love, and love is never wrong or "bad." They would in that situation say "feeling bad is natural." The expression of grief might be painful to observer or participant, but making our pain beautiful is about the consciousness of Grief as Praise and PROOF of Love. It's natural, and there's nothing 'bad' about it.
So while bad & natural does its tapdance in the thunderstorms of life, there is a tornado brewing, churning up rainbows in its swirling course until the spindle absorbs and then radiates every color imaginable. Tears become Holy Water, and the moistened, earthen heart sings its Praise.