First post here so I will give it a go...
I noticed a lot of people complaining about the reason why Obito started the war (death of Rin), but I think that is only the surface of the entire event. Madara clearly knows Obito, and I suspect that he did indeed save him that day under those rocks and used the Senju cells (much like Danzo did with his arm) to save Obito. Clearly, Obito is on an entirely different skill level then when he was with Kakashi, and it is starting to become clearer that Madara took him "under his wing" so to speak. During this time period, Rin died and Obito, being extremely angry and upset, allowed himself to join Madara fully. Madara used Obito's anguish to drive him into carrying out his plan, even after he died. Madara knew he would be dead long before the start of the 4th war, so he needed someone to continue his work and to bring him back with the Edo Tensei.
It was never Obito's war, he is in it purely for revenge for Rin, and that is the sole reason for him switching sides. This is most certainly Madara's war, a war he started literally from the grave with his only pupil pulling the strings until his revival.
I am certain I have extreme holes in this guess, but I feel as if that is what the Manga is leading to... eventually. All in all, a confusing chapter, but exciting at the end.
I was thinking that but still, atm in the manga he gave his reason so I'll argue that reason x)