I nursed (and pumped) exclusively for seven months. The first four were awesome, but then I got walloped with depression and lots of tears, frustration, an ill temper... you name it.
At my wit's end, I started seeing a therapist for PPD. Around the start of month 7, I stopped pumping during the work day and only breastfeeding at night (Co sleeper) and... Ta-Daaah! All those bad, im-gonna-lose-my-mind feelings disappeared.
I think there is something to be said for more in-depth research into all the postnatal hormones and physical-chemical reactions that go into BF.