In the fifth century B.C.,Pindaros ,the ancient greek poet,wrote such a poem in his book of Odes of Condolence ：thus every dog at last have his day —— He who this morning smiled,at night sorrow.I think you realise the dog in the poem doesn't refer to real dog,it is a metaphor that the poor people he.But the words has handed down in this context,and today we often say that a proverb ：every dog has his day.That is to say "everybody has his day" and it is similar to our chinese "shoe is on the other foot""the worse luck now,the better another time".
When your friends accidentally miss the chance,or passed successful,you mus to give him a powerful arms,and tell him ：Take it easy,!You know what they say,every dog has his day and you will have yours!