There is a certain bit of pain you feel when you play the Legend of Zelda Majora's Mask, the idea of transformations spreads both pain and joy to the player, joy from the change we get to see as the player as we realise some new abilities, but pain because of what we put Link through. Once we decide to put a mask on what we witness is pure anquish from Link as his body transforms into a completely different shape and what I can only assume is the rush of sadness that is brought out from wearing the mask. This is exactly what Eiji Aonuma discussed recently in an interview with Game Informer, which mostly discussed the pain of Links transformations as well as the soul of the Fierce Deity Mask:
Here are the questions as well as Aonuma's responses:
GI: “Why does it hurt so much for Link to wear the transformation masks?
Aonuma: We’re talking about masks that were created to contain the memories of people who have died. Often there are things they really wanted to do before they left this world, so becoming them is actually really painful because it’s like hosting a really powerful spirit that’s coming into you.”
GI: “Whose soul is inside the Fierce Deity Mask?
Aonuma: The best I can give you is just a suggestion. The best way to think about it is that the memories of all the people of Termina are inside of the Fierce Deity Mask.”