It's natural to think of the person you love the most before you die. People are simplifying his motives too much.
Yes Rin's death was the trigger for Obito's downfall. But it's the same thing with Nagato, would he have become Pain had Yahiko not died? Or would Madara have conjured up Tsuki no me if Izuna and his brothers hadn't died? Or what about Oro when his parents died? You can't use that as an excuse to belittle Obito's motives.
Now actually onto his motives. It's not just the fact that Rin died, but rather the circumstances of how she died. I'm sure if it was a regular mist ninja killing her, then Obito would not have flipped out as much as he did. Obito put himself through hell during his training in Madara's cave which lasted for at least a year. He dreams about how he and Kakashi can work together as a team now to protect Rin and who does he see with his hand through Rin's heart? Kakashi. Obito witnessed his own friend killing his other best friend which is incredibly traumatising for someone of only the age of 14. People telling Obito to move on cannot possibly imagine an event like this especially when it's justified back in Konoha
No matter how you look at it, Obito's team meant the world to him. This is clear when Obito throws Kakashi out of the way of those boulders and his determination to save Rin. This is understandable as the Uchiha Clan cast him out for being a mere black sheep. Rin was the only one who actually acknowledged Obito and spurred him on to become to the Hokage.
Obito's motives are to end the ninja system. The fact that the Mist Village went as far as kidnapping Rin and making her the Jinchuuriki shows how far a Ninja village can go just for simple revenge. The shinobi world is corrupt and Obito wants to end that completely in his new utopia.
