Macbeth is jealous because he wants to be the king of scotland

He tries to kill the king of scotland to get rid of his jealousy so he blames the murder on Duncans kids

He is also jealous because he wants to be in front of the kings children

These thoughts twist his mind so as a result of this he becomes ruthless just like his wife

So he is determined to get the throne and nothing will stand in his way

I have been jealous before e.g. wanting a talent someone else has (E.T.)