KHR: But why?

Childhood

Growing up, Natsume got all the love from their parent. Iemitsu and Nana played with him cooed him, feed him and so much more spoiling him rotten on the way. While all the love Tsuna got was Yuuto's. Yuuto could be close and raise him for such little time because of his job as head butler for the Sawada family. He could only feed Tsuna twice a day in the midday and night when he got some spare time from his job. Tsuna was quite small because of the fact that he didn't have any milk from his mother, only eating porridge with some meat and vegetable blended in.

Eventually Natsume turned two and had his birthday. Birthday of Natsume was a time full of joy as the boy received so many expensive presents from his parents. He even got a big birthday cake that he would surely couldn't eat all of them 

"Arigatou Papa Mama, best birthday ever!!!!" said Natsume while smiling so brightly at Nana and Iemitsu. Every family look into the scene would defenitely be jealous of the Sawada. But if they look closely, they would see a smaller boy peaking at the couple and Natsume with a sad gaze, a butler scolding and trying  hard to take the boy back to the room and close the door tightly. 

That's right. 

That poor boy is Tsuna, the twin of Natsume. He had no birthday, "Birthdays" to him are just normal day Yuuto had said that. Only on that day did he get to spend more time with Yuuto and just stay in the cramped and narrow so called room of Yuuto.

Timeskip 

Later at night, Tsuna was woken up by the sound of door closing. Being a curious 2 years old kid, he peeked out and saw a strange man with blonde hair handling Yuuto something made out of metal and heard he said

"Here's my little "gift" for that boy. From this till the day you leave this household, train him like a slave he is or you know what's the consequences". Tsuna did not understand the sentence completely but hearing the word "gift" in it made him very excited. Yuuto told him once that today what the boy who lives in the mansion received "gift". And that they are given to people only on special occasion. 

After the tensed conversation Yuuto had with Iemitsu, he was excused back to his room. With a sad almost painful smile, eyeing the metal collar in his hand  he returned to his room. Opening the door he was caught off guard when Tsuna suddenly jumped at him, demanding what had happened and was it true that he receive present from that strange man. That night, Tsuna learned about his inevitable fate and that he was born as a slave. (Well of course his true family was left out because of Iemitsu request) After that day, he was taught everything that would later proved useful for serving the Sawada family. From that day, he was forced to wear the collar, a symbol of a slave. 

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And that's the wrap. Tsuna never has a birthday because well Yuuto doesn't technically have salary that is being paid monthly or daily. He worked as the head butler for the Sawada family and have a long - term contract. Until then the Sawada family will provide everything to him (Food, shelter and other neccessities but no money) so he couldn't afford a small birthday cake for little Tsuna