Because they probably think your someone else. You probably got mistaken for a famous person...don't worry it happens all the time! 

People stop me and ask to take pictures with my 3 year old sons all the time. I took them to a nearby mall on Christmas eve and within 30 minutes we were stopped 3 times by people asking to take pictures.

My boys are asked so often that they really dislike having their photos taken now.

its one of the more polite things that a foreginer gets.
at least they talked to you instead of just stareing
as Dude would say it's good being a rock star

I've mostly been asked by my students. However, one time I was out window shopping with some friends, and someone wanted my picture because I was wearing a cute hat. I'm not blonde or tall, so I don't stand out too much, usually people just stare a little or do a double take.

Better than when they don't ask. Nothing pisses me off more than someone pulling out a camera and taking a picture of me, like I'm just scenery or an animal in the zoo.

i dont mind people asking people doing it without permission is the problem

I do get a lot of requests from Chinese persons to take a photo of me with them. 
If I am alone, and they ask, I have no problem with it, they seem excited and happy to have a photo with a foreigner to brag about later on, and it does not cost me a penny.
If I am with my GF, I would ask her if she minds, and if she says no, I will agree.
BUT what pisses me royally is when they pull out a camera and point it at me and take a photo.  If I have time to react, I will turn or cover my face.  I have already worked a "reply" with my girlfriend, that my religion prohibits photos of me taken, because a photo will take my soul away and I will die soon.  You will be surprised to know that almost 90 % of the time, they apologize and delete the photo.
Also, I have met some very nice persons that way, and do have a few good friends whom I met by the photo taking requests, and been invited to many teas, bars and restaurants as a result of a couple of pics taken, and a nice smile and a few kind words.

I don't mind. I ask for pictures with women. I had a group fo women ask to take pictures with me on the Great Wall, when it was the last old ladies turn , I kissed her as they took the picture, made everyone laugh

Why? Because we are either breathtakingly beautiful or drop dead handsome of course!

Yeps they like takin photos of my adorable kids too! But, fine by me  as long as they ask.

If they ask I usually oblige. If they don't ask why would I care, it's only a fkn photo so no skin off my back. I just don't get all the angst about this and the staring stuff. It has never bothered me because I just don't take any notice of it.

Grow your hair a little long, get some 3 day growth on your beard and walk around with Bono glasses at night....then charge 20rmb a shot.

Cause we all just wanna be big rockstars
Livin' in hilltop houses driving fifteen cars
The girls come easy and the drugs come cheap
We'll all stay skinny cause we just won't eat
And we'll hang out in the coolest bars
In the VIP with the movie stars
Every good gold digger's 
Gonna wind up there
Every Playboy bunny 
with her bleach blonde hair
and well..
Hey hey I wanna be a rockstar
Hey hey I wanna be a rockstar

.

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Chinese brain cell in general are pretty simple and they apprecite positive things. Simpleness is easily  to make ppl curious to many things if they haven't known/learnt it before. Beauty is a positive thing. In many of their eyes, other races are kind of the most curious thing for them, and some beautiful races even make them more interested. This is the reason they like to take picture with foreigners, simply for satisfied their curiousity, simply for fun (if during a trip), and it's very friendly, not simply for brag about it later on (they talk about it but not it's something important for them at all)

I think every guys like the chance to be close with a beauty. It's kinda same...many beautiful Chinese girls had experiences to be approached by all kinds of different races guys here or there...Some average guys really like to take pics with a hottie, so he can brag about it later on to his buddies..etc....ahh...off topic again!!!