A tragedy is the recounting of the tragic events leading up to the fall or death of the protagonist, who starts out on top and takes a tragic downfall. Therefore Shakespeare's Macbeth is a tragedy because it traces through many events, the fall of Macbeth, who starts out from a high position.
Macbeth starts out on top when the Thane of Glamis dies making him the new king. In hearing his prophecies, one of which being "All hail, Macbeth! Hail to thee, Thane of Cawdor"(1.3.49) he wonders how it could be true. At the thought of killing the king he "yield to that suggestion Whose horrid image doth unfix my hair"(1.3.134-135). He obviously thinks that killing the king would be a bad idea and does not want to do it. His thoughts on fate, "If chance will have me King, why, chance may crown me, Without my stir"(1.3.144-145) is showing he does not want to take fate into his own hands.
These are some examples of how Macbeth starts out on the top. But he is in danger of falling because of some of the decisions he may make.
Another way the reader can tell that Macbeth is going to fall is when he starts keeping stuff from people and does not tell anybody what is going on. One example of this is when Banquo tells Macbeth that "I dreamt last night of the three weird sisters"(2.1.20) Macbeth says, "I think not of them"(2.1.22) when in actuality he has thought of them and what they said. Similarly Lady Macbeth does not tell anyone the way she feels "'Tis safer to be that which we destroy Than by destruction dwell in doubtful joy."(3.2.6-7) and acts completely different around other people.
Other characters assist in the downfall of Macbeth by influencing...