Kakashi is the reason Obito became what he did!
Many people think that this is the reason Obito did what he did. I cannot blame anyone who believed in this or still does, because at one point, so did I.
The good news is, it is not! Kishimoto will never use something as trivial as the death of a fellow comrade in battle, albeit he loved her, as a deterrent to start a war and bring an end to the world as we know it.
The reason is Kakashi and his big mouth!
Look here. Throughout the manga, and this has been strongly hinted by Kishimoto, Kakashi has made promises he didnt keep.
It would take a while to list them all, so I will give you some examples.
The first time Kakashi promised anything in the manga or gave his word, he said he would never let his comrades die, a few minutes later, his comrades, a bunch of kids just out of the academy, had to fend for themselves against the Demon of the mist, while Kakashi was trapped in the water prison.
Kakashi promised Sakura, everything was going to be fine, and team 7 would return to normal. Well....did it?
Kakashi said he would take on Sakuras burden and kill Sasuke. Did he? He was actually going to get himself killed, if Sasuke hadn't lost his Sharingan right then.
Right after, he said he would take care of Madara with his Kamui, and a simple hand gesture from Madara(at the time) was enough to make him doubt himself.
There are more instances like this, but you guys get the picture now.....
It has always been hinted that Kakashi, although an elite, has seldom kept his promises.
Well, in Obitos case, he is going to wish it was the one promise he would have kept. Because that is the reason, Obito does not believe in this world anymore, and wants to create his own world which is free of lies and failures!
To support this as the only truth even further I would like you all to look at the following page:
"you know this world holds nothing but lies"
Do you see and agree?