Of course the obvious reason is that they hurt. (I’ve heard this from other sources — I have not yet submitted my double-Ds to it, and don’t intend to until I’m 40… if then.) But my cousin’s experience highlights another reason. She found a lump in her breast, and they did an ultrasound of it instead of a mammogram. She’s just 35 so she hasn’t had a mammogram yet, but this was the second lump she’d found (the first was benign). She had a large cyst in her breast, which would have been squished and possibly would have… (there are probably some technical terms for this, but I’m just going on what my mom said her sister said that my cousin said) broken the cyst and/or made it spread.
I’ve heard some people being concerned about mammograms for this reason — that it may compress the breast and squeeze the cancer in such a way as to make it spread and make it harder to get all of it; but I’m pretty sure this cyst was benign, like the first lump. Still, it needs to come out, and since she didn’t have a mammogram, it will make it easier to get it out in one piece. Other people don’t like mammograms for breast cancer because, after all, they use X-rays which are known to be cancer-causing. It does seem stupid to me to use a cancer-causing procedure in trying to prevent cancer, but I know that it is standard practice, and may be the most accurate means that we currently have.
Still, it would be good to ask your doctor about other options, including an ultrasound, to get an image of the breast. I vote for that, simply because it’s less painful. You may end up needing a mammogram to be most sure or most accurate, but in some instances ultrasound is a perfectly valid alternative.