"According to the prophecy recounted by Melisandre, after a long summer, Azor Ahai will be reborn amidst smoke and salt wielding the flaming sword Lightbringer to defend the world from R'hllor's nemesis, the Other. While Melisandre is certain Stannis Baratheon is Azor Ahai, others are not so sure."
http://awoiaf.westeros.org/index.php/Azor_Ahai/Theories#Jon_Snow"Jon's wounds smoke while Bowen Marsh cries, providing the prophecised salt, and Ser Patrek's bloody, star laden heraldry is in the air above.[5]
When Melisandre looks into her fires for Azor Ahai, she sees only things to do with Jon or snow, which he thinks must just be ash but she sees a possible connection to Jon.[6]
Jon Snow has a dream where he fights the Others with a red sword burning in his hand.[7]
If having the Blood of the Dragon is a prerequisite, if Jon's father is truly Rhaegar, Jon would satisfy that prerequisite. He would be the grandson of Aerys and Rhaella and thus come from their line.
Jon chose his duty to the Night's Watch over his love for Ygritte. He indirectly caused her death by warning Castle Black of the incoming wildling raid. Similar to Azor Ahai sacrificing Nissa Nissa to complete Lightbringer."