Because a lot of the fans' theories about Tobi's true identity were a lot better thought out than the revelation itself.
My name is Obito Uchiha. I am not Madara, though for a long, long time, I pretended to be.
My name is Izuna Uchiha. I am not Madara, though for a long, long time, I pretended to be.
My name is Shisui Uchiha. I am not Madara, though for a long, long time, I pretended to be.
I shouldn't have lived after I gave my eye to my best friend. If I had died then, I wouldn't have had to see him again, with his hand halfway through the chest of the girl I love. I wouldn't have had to see her crying as the light left her eyes, stolen away by someone who promised he would never turn his back on her.
I should not have lived after the younger Senju bested me in battle. If I had died then, I would not have had to see my brother, my beloved older brother, exiled by his own clan, by his own village. I would not have had to see the incorrigible Senju, pretending to be far mightier and vastly superior than they really were as they rewrote history to turn him into a blemish that stains the reputation of their perfect village.
I shouldn't have lived after I jumped into the ravine. If I had died then, I wouldn't have had to see my best friend manipulated into becoming a criminal, a murderer, for the sake of the village he loved so much. I wouldn't have had to see his brother left alone and ignored as if he had died with the rest of his clan, with too much pain and anger and despair for any one man, let alone a seven-year-old boy, to bear.
It wasn't so bad, working under the real Madara Uchiha himself. He can actually be pretty nice, despite what the stories say. He saved my life and took better care of me than my own family ever did. He told me such amazing stories of the village's early days, of his more-than-best-friend, his more-than-brother. But no matter what, he never let me forget the pain I felt the day she was killed. With his urging, my pain became my weapon.
For almost a whole century, I waited. I studied, even when I only had one eye with which to do so. I trained, even though my body was old and frail. Always, endlessly preparing. I kept my wits and my reflexes sharp because there would surely come one day when I would need them. And the final nail in the coffin – I adopted my brother's name, because he is not a monster like the myths depict him. He is not the villain history paints him to be. He is history itself, and if he is unable to make that known to the world, then I will do so in his stead.
"Everything's going to be fine," I once said to my best friend – but everything wasn't fine, and it was my fault for being unable to fix the situation. So I had to save them both. I had to save my best friend from the life of an S-ranked criminal that belonged to everyone but him, and I had to save his brother from a life of pain and misery. They were the only two people I had left who mattered, after all.
I've already tried to avenge her in the little ways I can. I made Kirigakure, one of the guilty parties, feel my wrath. I crippled them by taking away their bloodline abilities. I made the two worthless pieces of trash that are my sensei and my so-called best friend suffer. It's their fault for hurting her – not mine for giving them what they deserve. But it's not enough – that's why the Infinite Tsukuyomi is perfect.
Amongst the generations only slightly below mine, my brother's name inspires shock and fear, as expected. But the younger generations respond only with confusion. Were my brother's efforts worth that little? Was he so insignificant that our descendants cannot be bothered to even remember him, let alone honor his legacy? My brother's legacy extended far greater than any of them know – that is why the Infinite Tsukuyomi is perfect.
My goal is the same. Though my hands are covered in blood, it's all to bring peace to the world. My best friend has died, and his brother is now my responsibility. A world of peace and happiness will serve him better than that damned village ever did. But it's impossible to create a world where everyone else can be happy along with him – unless the world itself were to reflect everyone's deepest hopes and desires. That's why the Infinite Tsukuyomi is perfect.
It will make the pain go away.
All generations will experience my brother's greatest discovery first-hand.
Everyone will be happy.
So I fight.
So I fight.
So I fight.
Because I can't live in a world without Rin.
Because Madara deserves justice.
Because I need to make it up to Itachi and Sasuke.