This quote relates to one of the principle themes of this book, which is that of death and its acceptance. The quote relates to the author's mother and father, and how he is slowly coming to accept the death of his parents and move on in his life. The "thoughts" that the author says should not be thought of are those that focus on darkness and pessimisim, whereas, in an excellent analogy, he describes his stage of grief as being able to look at a black sky, and pick out to bright shining stars to represent his parents. Rather than focusing on the grief and inexorable sadness of the situation, therefore, he is able to move on and value his parents' lives for what they represented and who they were. We can only stay despondent for so long.
This quote therefore represents the moving on of the author as he begins to move away from grief and sadness and is able to contemplate his parents and their death in a different way. He moves from seeing the blackness of the night sky to being in awe at the beauty of the stars.