S.H.
OMG!
I thought my son was the ONLY one like this!!!
He LOVES "my" armpits!
He is 5 now... and still loves it. Although it has gotten less in frequency and he is not as hung up on it now.
He started this thing from about 3 years old, too.
I ask him why he likes my armpits, and he says its "cozy" and he likes the "pokey" feel.
Its sort of a soothing thing for him.
Anyway, it does not frustrate me. My armpits are not sensitive.
My son, also likes to rub his hand on my Husband's face, when it is stubbly and he hadn't shaved. Because that also has the same "texture" as *my* armpits.
But, what we did once, was got him a scrub brush. Which is "pokey" in feeling too. And he likes to rub that.
My son will even tell me not to shave my armpits. But I tell him I have to... or I will look inappropriate.
He doesn't battle about it.
Try giving your daughter, something else to rub. Something with the same texture as your armpits. As I said, we got my son a scrub brush. One that he liked. And we told him why. Still though, he likes my armpits. But he'd carry around the scrub brush.
Too funny.
Or to get my son to stop... I tell him I have Deodorant on... so he must not rub it now.