Life isn't fair.
When people said this to me when I was younger it would really frustrate me. I took it to mean that they were pessimistic and believed that life was difficult so suck it up.
I think about the phrase differently now. Life really isn't fair. Life is unpredictable, convoluted, intense, random, interesting, confusing, baffling, challenging, exciting, difficult - but not fair. This doesn't mean that you should 'suck it up' or just accept what happens. It just means be prepared. Be aware that things won't always work out as you would expect them to, nor will they seem just or right all the time.
That is the essence of life and it is the unfairness which allows us to grow. If life were fair it would be dull. You would never be challenged to stand up for yourself, embrace change, and take life by the figurative horns and show it that you're up for whatever it has to throw at you.
Life is, however, balanced. I recently watched the very delightful and well cast 'The Best Exotic Marigold Hotel'. Even if you haven't seen it, if you've seen a preview you've probably heard the line:
"Everything will be alright in the end. If it's not alright, then it is not yet the end."
There is a wonderful Buddhist story that often gets told about a family in a small village. The mother and father live there with their son, who grows to be a handsome young man. The couple are pleased as it means he will be able to care for them in their old age. But one day the son is thrown from a horse and crippled, confined to a wheelchair. The parents lament his poor fortune, mourning for his strength.
One day a General comes through the village. The country is at war and all able bodied young men are being called to go fight. The couple are relieved that their only son is able to stay with them, rather than be taken away to fight and almost surely be killed.
This is a demonstration of how something which seemed unfair came to have a balance in it. Sort of like 'be careful what you wish for.' But the story continues on. The next situation could reveal that the family is distraught because none of them can work and in the time of war they seemed doomed to starvation. But then it can continue still and because of their son's confinement they are given food and care by the other villagers.
The point is that life just happens. How we deal with it is up to us but what we have to deal with is never predictable nor planned. But in the end, there will always be balance.