Part 1

Doflamingo was born with some sort of illness which makes him blind in the left eye and that forces him to wear a pair of sunglasses because if any other Celestial Dragon would see then Doflamingo would immediately be discarded, because they can't afford any flaws to their nobility.

But despite this Doflamingo did live like any other normal Celestial Dragon did, like wanting to have human slaves and wanting them to kneel down before him. So yeah, he had all that traits of a cruel Noble. But somehow his father and mother were different and that maybe because as the formation of Celestial Dragons goes 1000 years back so during this timespan some outsiders might have also been included in the Donquixote Family which by nature weren't so cruel or crazy but instead were kind and humble and didn't usually demanded any human slaves or did any disturbing deed. And this also went down to his brother Donquixote Rocinante.

Since the start Doflamingo is shown as a bright and a clever kid who also deeply loved his family especially his mother. He usually looked up to his father with pride as Homing held a significant position among the Nobles of Marijois mainly because of family status. But all of this happiness and luxury was soon turned into a complete sorrowful tragedy.

To prevent other Nobles finding the problem with Doffy, his father Homing moved away from Marijois. All that he said was that he wanted to start a new life which was to live among the common people. But he never told the real reason to anyone including his own son, which gives Doflamingo the idea that his father is 'stupid' and that he's leaving the noble land where they could get or do something with just a snap of their fingers. So, I guess this was the very first stage where the little negative sting was born in his heart against his father. But I guess except Doffy, his mother and brother also knew the actual reason but they didn't tell Doffy as they knew that he would blame himself for it and would insist to leave him instead of moving all by themselves.

Thus, they left Marijois having a deal that they would never return back but in exchange they would arrange them a house and other necessities to live.

Note: It's my very first try so please guide me or correct me if you find me wrong somewhere. Will appreciate the support!