Its true that Obito is not the only one to have lost his comrades/love-interests/friends but all the other characters had no choice but to cope with it whilst Obito had an extra option, extremely selfish one at that .. hence the Moon Eye plan. He was still a teenager when all of this happened and it seemed like Rin was the only person in Obitos life that he cared for (maybe Kakashi a little bit as well) bcs we didnt see any other of his friends or even parents. I assume he was an uchiha orphan, much like Naruto.
However, I have a quarrel with one thing. Why not revive Rin with Rinne Tensei ? If Madara could be revived after such a huge time-span then there should be no problem to revive Rin as well. Why isnt that enough for Obito ? Or rather, isnt that what he wants ? Bcs I dont know anymore since he is cool with having just an ilusionary Rin instead of the real one. Its like he doesnt want to save her anymore but only making himself feel better .. just pure selfishness.
There are a very limited amount of options when one teammate kills another, Obito must have decided that he didn't want to go about living a life where Rin MIGHT have always been a traitor.
Also, most bad guys are selfish. Obito nearly died and wanted to see Rin and Kakashi again so very dearly. In fact, his only reason to go on living was to see Rin specifically.
To see her die by the hands of someone he entrusted to her, knowing that Kakashi wouldn't kill a team mate unless he absolutely had to, would predictably make him go mad.
He was under a lot of pressure and stress when he woke up in that dark cave, and this was the last straw. He went berserker.
Uchiha are very latently powerful ninja, no matter how long it takes them to awaken the Sharigan. Add the 1st hokage's cells and you have a powerhouse of a ninja.
You guys all seem to complain about Obito's motives and how he got so powerful in such a short time. You all seemed to have forgotten how many characters went absolutely nuts when some one close to them was hurt. You've also seemingly forgot that ninja corpses are very important in the Naruto world. That's why the Anbu exist. to make sure that techniques or abilities in certain ninja clans don't fall into enemy hands. So having the power of the 1st Hokage will naturally make even a butterfinger Uchiha like Obito immensely powerful.
Also don't forget that Obito did have a great teacher, and most likely trained along with Black Zetsu to grow even more powerful.
Becoming more powerful is not limited to a great length of time, merely just how fast a person can grasp the concept of a technique.
Obito's new resolve would have motivated him to learn much more quickly and his enhanced body would have helped him bypass a lot of the regular training most other ninja's would have to go through.