Rin's death was the catalyst, so its not surprising ....... Just like Yahiko was for Nagato, but Nagato sounded much cooler with his "Pain this.... Pain that ....."
If Rin was apart of the manga long enough for us to have some type of connection. That would have been a lot more motivating, but no one cares about Rin. Obito is losing his mind. Thinks about Rin and his mind comes back. No, Kishi. No.
Obito cares about her. That's enough. And the Obito-Rin-theme has been around for about 50 chapters now. We know well enough by now that she is at the core of everything he does, everything he is. I like it.