Here the quote, "Our doubts are traitors, And makes us lose the good we oft might win, By fearing to attempt." is said in William Shakespeare's play, Measure for Measure (c. 1603). It is a line said by Lucio to Isabella in Act I, scene iv.

Many scholars and critics have discussed the meaning of this quote. They agree that when people start to second guess their actions or do things out of fear, they lose the opportunity to do something great or experience something good. So when writers and others like Shakespeare say: "Our doubts are traitors...." they are saying that our inner voice may work to betray you. It may cause us to lose self confidence and courage to take any and all appropriate actions.

More Info: shakespeare.mit.edu