Watanuki doesn't love Himawari. Yes, he likes her, but not romantically, and only acts the way he does because she is safe. She is not involved in the spirit world (at least, as little as someone who is involved with Watanuki can be): she is not placed in the line of fire, or goo, or water, or darkness and spiders and curses and blades and ayakashi and youkai and whatever else he and Doumeki encounter on a daily basis. She is safe because she doesn't like him back except as a friend - or if she does, she cannot, does not, express it because she too has reasons, like Watanuki has reasons. Which is why he feels very little at the thought that he is losing her - only a small echo of a pain in his heart when he realised that she wasn't there at his side.
He doesn't think too deeply about his reaction to the thought that he could lose Doumeki as well.
Watanuki doesn't love Doumeki. Yes, he will now admit that he also doesn't hate Doumeki, and Doumeki has always been there when Watanuki needed him, and Doumeki's antics are as comforting now as they are annoying, and Doumeki is his anchor, but there's more to it than that. It is not that Watanuki does not want to love Doumeki - because he does not, no - but that he cannot. Because Watanuki is frozen in time, because Doumeki has no business binding himself with a being that cannot touch him, be with him, in any way, because if he cared any more about Doumeki he would break into a million pieces. Because if Doumeki was lost to Watanuki, and Watanuki did love him, Watanuki would probably cease to exist, because he doesn't think it's possible for him to exist without Doumeki at this point, and Watanuki promised. He promised he wouldn't disappear.
Doumeki doesn't love anyone. There are people he has gotten close to, people he wants to protect, but there isn't one that he loves. His situation is singular, being that he has no idea what to do. This is, of course, the first time this has ever happened to him, or perhaps the second. Or third. Or perhaps he's been unbalanced since the day he met Watanuki. It was... sometimes he'd the nagging sense that Watanuki was different, but he hadn't known just how different until he got to know Watanuki. It's a choice that he's never come to regret, except in one aspect. He's never given it thought, or rather, never given the thought enough attention for it to become fully fledged, because if he thought about it, it would become more real.
The thought comes back every time Watanuki puts himself in danger - and the thought has become Doumeki's reality.
Doumeki doesn't love Watanuki. For Doumeki, there is no one more stubborn, troublesome or infuriating. Doumeki has gone through rain and fire and hell - or something like it - for Watanuki, and he'd do it all over again. Despite this, despite everything he has sacrificed, he does not have Watanuki. Not in the least. Perhaps it is only proper: Watanuki was never meant to be in Doumeki's world. But Doumeki cherishes every moment they have together, he knows that Watanuki is on borrowed time, even as Watanuki slips through his fingers, almost as if he never was. But Doumeki refuses to let Watanuki go; not because of obligation or responsibility, not because of friendship or loyalty - it has never been about that. He doesn't know why, but for him there can be no other. All he can do is be there when Watanuki needs him, no matter how painful - torturous - his trials: he will stay.
Love is shallow compared to what they feel for each other.