I can think of many reasons for postpartum depression but they all come from the same source: a society that is unfriendly, if not hostile, to mothers and babies, from pregnancy on. Our pregnancies are STILL treated as a medical condition, to be "handled" by medical doctors and hospitals (leading to many unnecessary caesarians and other "interventions") when what we women need are midwives and mother-and-child friendly birth centres where births are  far more positive, natural, experiences than the ones taking place in hospitals. 

To focus on problems of breastfeeding and the supposed pressure to do so even when there are difficulties is to be barking up the wrong tree, unless this mother had particularly nasty friends and relatives who criticized her about her difficulties. The authors and the grieving husband would be wise to broaden their perspective to look at the predicament women and particularly mothers find themselves in the Canadian society, which is not a happy place.