If asked, Doumeki would say that his first impressions of Watanuki were that he was loud, spastic, and annoying. Those first impressions never really went away even though Doumeki got to see Watanuki as more than that. Watanuki was loud, yes, but as loud and angry as his words often were, his actions contradicted them. He was thoughtful and self-sacrificing, empathic toward others' problems, and hardworking. He didn't cut corners and made food that was full of good intentions. He saw too much and held more questions than answers. He bent to whims even if he protested the whole way and could be remarkably short sighted.
Watanuki inspired a weird amount of loyalty for someone that was perpetually on the edge of a meltdown. Maybe it was because he was awful about taking care of himself, or maybe it was because he would cook the foods Doumeki liked even if he did complain. Either way, Doumeki couldn't help but get caught in his pull. They were friends in their own way. Seeing Watanuki get pulled under into all sorts of spirit things, Doumeki couldn't help but follow and protect him. It wasn't something he could explain with words; words were never his strong suit. At a gut level, he knew he couldn't see Watanuki self-destruct.
After Yuuko, though… After Yuuko, Watanuki wasn't the Watanuki that Doumeki knew anymore. There was no flailing or rants, just long stretches of awful silence. Anger had never turned Doumeki away. Neither did silence now.