If you are watching Shippuden, dont read this post jaja
I will talk, Why the masked man begin the fourth great war shinobi.
The people who have seen Shippuden know that the masked man is Obito. To put in context Obito was a young Shinobi who belonged to the Minato team, his teammates were Kakashi and Rin. Obito always was in love of Rin. In the thrird great war Shinobi in a mission of save Rin, Obito "died" crushed by a stone. Obito always wanted the peace in the world Shinobi, After a while Kakashi killed a Rin because She had information very important about Konoha that it could not be revealed.
The main reason he started the war was so that he could create an ideal world. Rin's death led to Obito realizing that there will be no peace in this world unless he does something about it. After Madara told Obito about the plan, he passed away leading to Obito being the only person who could execute the plan. He wanted for everyone( Rin, Kakashi, Minato etc.) to be with him and live happily forever. But Rin's death made him realize that there are going to be more wars and more people are going to be killed leading to maybe Rin's death once more.
