It is often easier to focus on the negative aspects of life rather than focusing on the positives.
Get your child writing and help them develop gratitude.
Every day, give your child three pieces of paper. On each paper he/she writes one thing that they appreciated about that day (it could be simply having breakfast). Keep the pieces of paper in a jar.
When your child is feeling that life is unfair, ask him/her to randomly pull out and read the messages.