Symbolism, Imagery, Allegory
Nurses are Oskar's fetish. First of all, his mother was a nurse; that's how she met Alfred Matzerath. And we know Oskar was a mama's boy. Also, because of his physical problems, he spends a lot of time in hospitals getting washed, touched, and cared for by the nurses. As he gets older, all this stimulation is, well, stimulating. The nurses are his friends, but he kind of gets conditioned to be sexually excited at the thought of them, and this ends up being his undoing when he becomes obsessed with Sister Dorothea. He's never even met her, but he's overcome with sexual longing for her. Nurses represent both nurturing and sexual excitement.