PPD.... (and breastfeeding.... or lack of)

I have encountered many people lately that have suffered greatly from PPD (post partum Depression) A running theme that I have often encountered is that many of the Moms are not breastfeeding and have never breastfed, or tried and it didn't work out for whatever reason (informed or uninformed) ...
I believe that PPD is very real and that it is something that many women go through... However, I also believe that it is diagnosed too much and that medication is given when it is not always the best answer... However, there is something else that I am starting to believe more and more...
When our child is born the natural thing is to breastfeed (Formula is not natural no debate could convince me otherwise) when a women doesn't breastfeed however, the body stops producing the hormones to make milk....(milk production is activated by demand which then makes the body produce hormones which develops the milk) Anyways, when these hormones are no longer in production the body actually think that the baby has died which makes the body in question start the process to become pregnant again...
If your body believes that your offspring has died wouldn't the logical conclusion be that sometimes the body reacts negatively chemically and therefore not breastfeeding could be a cause of PPD?
Another thing is that breastfeeding helps create a very strong bond between Mom and baby. This is why Breastfeeding is one of the fundamentals in Attachment Parenting. Couldn't it also be that this lack of bonding that is often a cause of PPD also come from the lack of not breastfeeding?
I do completely understand that PPD is a medical condition that can stem from many reasons or even no reason at all but I really believe that the lack of breastfeeding can often have a lot to do with it...