WARNING: This theory is a loose extension of a theory made by MattPatt at Game Theory. If you haven't watched his video, please do as I will reference several points in his theory.
The Legend of Zelda: Majora's Mask. A game praised for its dark tones and ingenious 3-day cycle. It has been theorized about by many, trying to unlock the secrets it holds. A newer, more popular theory states that Link is dead in this game, with the land of Termina being a purgatory for the spirit of the hero. And while there are definite flaws with this idea (the Hero of Twilight is the Hero of Time's descendent), the purgatory idea got me thinking. And after much research, I have deduced the following conclusions about Majora, the Happy Mask Salesman, and the entire game itself.
Majora is perhaps the most mysterious Zelda villain of all time. We know hardly anything about it, save that the mask itself was used for hexing rituals by a dark tribe until it was sealed away. Yet we see that the mask isn't a simple artifact, rather a demonic entity sealed away inside the mask with its own agenda and goals. It clearly exists in Hyrule, as shown in the opening of game, yet it tries to destroy not Hyrule, but Termina. Now we know that Link is the physical form of the Spirit of the Hero, reincarnated whenever Hyrule is in danger. Even if Link is killed, another Link will be born to strike back evil. So, with this knowledge, it can be reasoned that Majora launched an attack on Link's very spirit, trapping him inside Termina. In this state, Link is neither alive nor dead; rather, his spirit has been transported into this realm as a sort of shield against the attack on his spirit. Majora knows this and proceeds to destroy the realm and Link along with it, effectively annihilating Link's spirit and ending the cycle of reincarnation. With Link gone, Majora would be free to decimate Hyrule. This conclusion can be reached by logic and the following facts: Majora exists in Hyrule. Majora tries to destroy Termina, not Hyrule. Link is a reincarnated spirit. Combined with the idea that Termina is a spirit-realm for Link, and you have a very devious plot indeed.
So if all this is the case, how does the Happy Mask Salesman fit into all this? While many see him as sinister, I disagree and see him as an ally and mentor to Link. He obviously possesses knowledge and abilities beyond others in Termina, aware of Link's time traveling and Majora's powers. He even seems to have time abilities himself, able to send prevent Link's destruction at the last second by sending him to the first day. And his laugh? Perhaps it seems scheming, but I will agree that he knows more then he lets on, and is calm when others panic. But he is not evil; rather, he is a guardian angel to Link. He gives guidance about what must be done to Link, saves him at the last minute should Link not play the Song of Time fast enough, and even encourages him with the phrase "believe in your strengths". Bare in mind that the Hero's Shade, the ghost of the Hero of Time, uses the phrase also; this implies that he valued the Happy Mask Salesman's words of wisdom and help, and didn't see him as evil.
Now, I know what some of you might be thinking. What about the freak-out scene when Link doesn't get the mask back the first time? I say that he isn't angry or sinister, just afraid. He has been sent to guard Hyrule's greatest hope, and he will surely be punished if he messes up. For a second, he forgets that Link still holds the power to defeat Majora, and begs him to re-double his efforts. In a way, the Happy Mask Salesman serves as a mentor to Link, helping him grow spiritually and heal some of the doubts and sadness in his spirit when the salesman plays the Song of Healing for him. If Ocarina of Time was about physical growth and growing up, then Majora's Mask is about spiritual growth by bringing happiness to others, and in turn to oneself.
This brings us at last to Fierce Deity. A mysterious being, one who greatly resembles the grown-up Hero of Time, the Fierce Deity is a powerful being. He is described has having terrifying power and is the "bad guy" to Majora. Some have taken this to literally mean Fierce Deity is a being as evil as Majora. Yet consider how you obtain his mask: by trading in every mask in the game. Through the entire game, Link has helped heal the sorrows of others, bringing happiness and earning masks as rewards. The Happy Mask Salesman, when talked to while wearing one of these masks, remarks about the positive qualities they embody. Leadership, gratitude, and true love are all aspects embodied by both the masks and Link himself. So by collecting all these masks, Link earns the Fierce Deity mask. Physically, it embodies the peak of power that Link could become and the will of Link's spirit. But it also is the culmination of the healing and happiness that Link has brought to others and himself. Taking on the form of Fierce Deity, which I believe to be the manifestation of the Spirit of the Hero, Link defeats the evil of Majora and leaves his purgatory alive, a stronger Hero and a stronger spirit. This is, to me, the true meaning behind the symbolism in Majora's Mask. Behind all the darkness and tragedy is a story of a boy bringing happiness to others, a story we can all try to emulate.