The brain is an electrochemical machine. Your brain chemistry changes when you're happy, hungry, sad, angry, bored, horny, sleeping; pretty much every human experience is mediated through brain chemistry changes.
And in any other machine when something breaks you fix it. If your rotator cuff gets torn you don't say "just stop having shoulder pain" or "shoulder is a natural part of the process of life and you have to deal with it," you go to a PT, maybe have surgery, maybe even take a pill or two.
If you think PPD (or any other form of depression) is "brain chemistry" on the same level and getting hungry or horny you grossly misunderstand what depression is at a fundamental level.
The serotonin imbalance hypothesis of depression, widespread as it is in popular culture, has actually been pretty thoroughly debunked at this point[1]. We know that SSRIs do have a therapeutic effect, but it seems their effect is oddly enough unrelated to serotonin levels.
What they seem to do is mask the depressive symptoms to the point where you're given a sporting chance to "behave" less depressed, which is what seems to be the most effective treatment. It's fairly analogous to how painkillers can cure back pain by permitting you to move around more.
Not acting like you're depressed is the real cure, which is of course very cruel, because as anyone who has had a depressive episode will attest to, just getting out of bed may be a struggle.
