DISCLAIMER: I MAKE NO CLAIM TO HAVING ANY OWNERSHIP OF THE SERIES D. GRAY MAN. THIS WORK OF FICTION WAS DONE PURELY FOR RECREATIONAL PURPOSES AND NO PROFIT MONETARY OR OTHERWISE IS BEING MADE FROM THIS.
SPOILERS: THE ONLY MAIN ONE BEING ALLEN'S EYE FROM THE CROWLEY ARC OF THE MANGA.
I apologize in advance if I messed up Allen's characterization too much with this. This is just my interpretation of why he's always so cheerful and helpful despite everything he's gone through.
Everyone is so enthralled with the single shade of the world they see.
Where everything fits into neat little areas and sides are clearly defined by what is perceived by the person.
Even when things take on the grayer edges of a world unlike the one they hide in.
Safe on their side of black or white none know of the other side and those they are connected to.
Some though are able to catch glimpses of that other side were a simple choice drastically severed the line of normalcy.
Will you be one who shall fall into a world made of both black and white?
Monster, Demon, Cursed Child...
He had been called so many things by those who couldn't see what he saw. For so long he had believed them deep down in his heart that he was just that and nothing more. Nothing more then a Demon awaiting the day of awakening even as the part of him that was human struggled and yearned for recognition as nothing more than a child.
So he kept smiling and helping others.
Father Mana had been the first to not see or ignore the hand that made many recoil away from him in fear and loathing. He had been the first to say that his arm would not be what God would judge him on but on what he did in life. Mana was the one who showed that even if no one thanked him or smiled back that kindness was not something one should give up if they thought it thankless or of no use.
Even as the pain exploded in his eye as the curse of a damned was placed upon his soul deep down he knew that silently Mana was telling him to stay kind and never lose his smile. That day he learned how to smile even when he felt like crying out his pain to the world. A mask that wasn't really a mask for he always kept in mind that no many how thankless the task might be it, was better to be kind and hopeful then allow the pain and darkness buried deep down inside from taking over.
Still he smiled and helped those in need even if it was slowly killing him.
He still believed deep down that he was a demon even as those who came to call friends and comrades denied such a claim. That fear could not be alleviated as his eye evolved in the same way as the Akuma did. He didn't need to listen quietly as those who would accidently be pulled into seeing what he saw shuddered and whispered to one another of how it was like looking into hell. The pain they took to keep such conversations hidden from him did not go unnoticed by him, but because it made them feel better to think they were keeping words that may seem harmful away from him he let them think they were being secretive.
It didn't hurt him as they expect it would for deep down he knew it was soul and only strove to seek salvation for the sins he committed even as he strived to save those who had fallen under the same fate as Mana had.
He would keep smiling and helping until he could go forward no more.
Exorcist, Savior, Destroyer, Friend, Loved One...Allen.