Isaiah 43:18 Forget the former things; do not dwell on the past.
How often do you look forward to good times when you are upset about something? How often can you smile when you are sad?
God wants to be able to get us smiling at all times, not because we must ignore the sad times, but because we can look forward to the good times.
It is right to be upset during sad times, but when it is right we should put those times behind us and focus on the new things ahead with God.