Well Kakashi was the one to activate it. Now some ask why he didn't use it through his life or learned to use it on will. Simple this whole Tobi is Obito is just a huge plot hole, there was no need for Kakashi or Obito to activate the mangekyo the past chapter 605 shouldn't have been made, it would be better just with this one(606) his convection and change doesn't seem the least bit believable. I get he(misashi kisimoto) wanted to reveal to be tragic, dramatic, all that good stuff, but to me it just fell flat with plot holes and things that don't matter.