I never told my first two boyfriends about my fetish. I was too shy. Even now I'm often shy about telling a guy what I want, and just now have I really figured out what it is.
It's not just that being turned on by hair washing is different, but it involves having a guy do something for me that isn't inherently sexual, and unless he's into shampooing as well, isn't much of a turn-on for him. To ask for a shampoo is to ask to be massaged and pampered, an often selfless task for the shampooer, and perhaps that's why my last partner believed I had to earn washings, while he was more willing to have sex with me, which he did consider a favor to me even when it involved a good bit of coercion.
(yes, it's exactly what you think it means)
I wouldn't be surprised if other girls feel this way. It's quite an obstacle to overcome, feeling like you can't ask for what you want directly without coming across as too aggressive or downright selfish. I wish I had better advice to women on how to ask for it, but personally I've often had to be subtle about bringing it up. I've had to start with casually bringing up the topic of fetishes, and try to get him really interested in what mine might be before I finally spill it. Even as it comes out, I feel silly, embarrassed, as if my fetish is stupid, childish, and immature.
Luckily, most guys have found it cute, intriguing, and in a couple cases flat-out hot.
Most guys I've dated have seemed unwilling to shampoo me anywhere but in the shower. Maybe it's because that's the only way they can get something out of it - they get to wash a wet, naked girl and possibly turn the shampooing into a conventionally sexual act. I've dropped hints that I wanted it over the sink, they either fell on deaf ears, or they were intentionally ignored, or the guy made an excuse as to why not. Frustrating. Sometimes all I want is a shampoo, no sex or nudity involved. Why is that so hard for guys to comprehend? Well, I'm sure you guys get it, but not normal guys.