I woke up the other day too my cat grooming me in my arm pit region I haven't a clue a little concerned.. Sorry for grossing anyone out but it's VERY ODD behavior too me....
Sometimes they're drawn to the smell of deodorant. One of our cats used to be obsessed with "Old Spice" whenever my husband bought it...he be sitting on the couch and suddenly have a cat's head up his sleeve.
I would just be careful about it if you wear a long-lasting anti-perspirant (some are 24+ hours) there are just some things in them you probably don't want him ingesting.
Things that smell good to a cat or a dog often smell bad to a human and vice versa.
Dogs will roll in poop, not only as a throwback to times when dogs needed to do things like that to cover their scent, but also because they think it smells good. I've seen dogs that like to lick their human's sweaty feet because there is some component of a human's sweat that they like the taste of. Maybe it's the salt. Maybe it's something else. I don't know but I have known more than one dog that liked the smell of stinky feet.
I don't see why a cat would be any different.
Maybe it's the salt from his perspiration. Maybe it's something that comes from his sweat glands. Like others have mentioned, it could be his deodorant or his cologne.
Things that smell dirty and rotten to you might smell like Chanel No. 5 to a cat or a dog!