Asuramaru: You always find yourself in situations you can’t overcome unless you become a demon. Must be your karma.
[Yuu: What’s a karma? and later Asuramaru says: I only know what you know. Because I’m you. And Yuu answers: Don’t lie, Asuramaru. I didn’t know the word ‘karma’ earlier. You possess your own will.]
The dire straits they’re in just don’t give either of them much choice. If, for argument’s sake, Mika had his way right now and the two of them managed to run away, he would be able to save his precious Yuu-chan and maintain what’s left of his humanity for the time being, but at the same time, Yuu wouldn’t be able to retain his, knowing that he had betrayed and abandoned his comrades. They’re pulling in the opposite directions, so first they need to find a compromise. Luckily, Yuu is charismatic enough to possess a potent power of persuasion, and Mika is sensible enough to be willing to believe in Yuu and compromise, so they quickly found the common ground in the beginning of the chapter.
But if, say, Yuu becomes a full demon, Mika might be be even worse off than Crowley, because not only would he lose his precious person, he would probably have to kill demon!Yuu-chan with his own hands, and it’s obvious that it would destroy him emotionally.