If we had nothing to believe in, we would have a sad world and such an empty life. We see evil and it might sometimes seem to go unpunished but no one knows what the real end will be.
Life is not fair - we are not born with a guarantee that life will be fair or that all of our expectations will be met but sitting around complaining that life isn't fair certainly will not help change it, taking charge of what we expect and want might profit us better.
Bad people as well as good people get hurt, bad people might seem to be rewarded but again we cannot foresee the ending to what will happen yrs down the road
Children get cancer as well as teens, young adults and the elderly - even our loved pets get cancer - again no guarantees were given about life itself.
If all that was needed to bring us joy and happiness was cake and a furry friend life would indeed be sad and boring. Life's challenges are there for us to choose what we do with them, we are the masters of our own destiny.