In Act Five, Scene 5, Macbeth learns that his wife has died and feels indifferent about her death. After receiving the news that Lady Macbeth is dead, Macbeth comments that the news of her death was bound to come eventually. Macbeth has given up hope and has become callous to the situation at hand. He speaks indifferently about death by saying,

"Life’s but a walking shadow, a poor player that struts and frets his hour upon the stage and then is heard no more. It is a tale told by an idiot, full of sound and fury, signifying nothing" (Shakespeare, 5.5.24-28).

Macbeth believes that life is nothing more than an illusion and compares his existence to a poor actor who hopelessly wanders around on a stage for a brief moment. Macbeth is extremely depressed and feels like life is absurd and pointless. He reflects on his unfortunate situation and determines that life is void of meaning. The death of his beloved wife, coupled with his guilt and mental agony, have left Macbeth feeling numb and indifferent. Macbeth has given up hope and does not care about life anymore in Act Five, Scene 5. 

When Macbeth learns of the death of Lady Macbeth he is struck by the meaningless of life. He feels our lives don't have a lasting impact on anything. He refers to life as "a walking shadow" and a "poor player that struts and frets his hour upon the stage and is heard no more". We live, but not really. We are ineffectual players (actors) who take the stage (live) for a brief while but whose power and influence dies when our hour (lifetime) is done. Macbeth has no hope for a happy afterlife, and he realizes, with Lady Macbeth's death, how empty all of their moments truly were. Life is a trivial, senseless story: "... a tale told by an idiot, full of sound and fury, signifying nothing."

Out, out brief candle.

Perhaps it's Macbeth's way of coping with his wife's death, perhaps it's his way of coping with the knowledge that he's in a lot of trouble, perhaps it really is how he feels. Either way, Macbeth expresses a sentiment that life is short and then you die. I would venture a guess and say that life is meaningful for him which is why he killed Duncan in the first place. If these thoughts are really how he feels, he probably felt like he had to expedite his route to the throne in order to get the most out of it before his own death.