Lately, I have been working with several clients who have breast cancer. The one link I've noticed with all of them and other people I have known who have had it - is broken hearts. It can be a broken heart about anything: childhood, divorce, low self esteem, death of a loved one, even from past lives. I know from experience with myself and clients, that it is often the sad stories we keep repeating to ourselves and others that keeps that grief in our energy fields.
I once heard a woman on the radio talking about being diagnosed with early stage breast cancer while she was mourning the death of her mother. She was meditating one day and heard "build a monument to your mother on your property, not in your breast". She did just that, as well as other treatment, and the cancer went into remission.
I recently worked on myself using the Body Code and asked if I had anything to release that could lead to breast cancer and what came up was all the sadness I am still carrying from my dog dying and sister moving last year. Since I have already released a lot regarding that, there wasn't much to release. However, I realized how important it is to let those losses go as soon as possible and clear any emotions that are still in the energy field because once they move into the body, they can lead to disease.
I think the reason this sometimes manifests as breast cancer, and often in the left breast, is it's proximity to the heart. So keep your hearts happy and release any sadness you're holding onto!