They loved each other. Simple as that. Zabuza thought Haku looked like a girl and he loved him. Besides compared to how powerful Naruto and the others got they would be no match for them if they came back to life which they most likely won't.
Maybe Zabuza just had a soft spot like all "big strong men" have.
I mean come on... Who can pull the big strong act without having somebody they have a soft spot for?
Someone's God i think the cat girl's right, when you think about it makes perfect sense, you can easily think several tough guys and the someone(thing) they'll break an arm over.
Life is but a dream.....So what happens when you wake up?