When toddlers first learn to ask the question, 'why?' they drive their parents to exasperation. No matter how carefully the adults attempt to explain, the question just bounces back. 'But why this?' And, 'Why that?' The children only really ever stop asking because eventually they realise that the question 'why' simply never elicits a satisfactory response. There is no answer to the biggest 'why' in your life, but as you begin to become happier about a situation, it won't bother you anywhere near so much. 
Sometimes you have to forget what's gone;
Appreciate what still remains;
and look forward to what's coming next...