If you are sad because you can't have something you want - maybe a book or a toy - you can do one of two things: you can do your best to get it, or you can stop wanting it.  Either way, if you succeed, you won't be sad any more. 

If we can stop ourselves wanting all the beautiful and pleasant things in life, and can learn to control our greed for happiness, comfort, recognition and affection, we shan't feel sad any more when, as so often happens, we fail to get what we want.  He who ceases to wish for anything ceases to feel sad.  If the appetite goes, the pain goes with it.

Gombrich, E.H., A Little History of the World, p55.