Is there any psychological explanation explaining why people can get postnatal depression?

Its physical: Adderall (dextroamphetamine and racemic amphetamine) mimics the "flight or fight" response both psychologically and physically. When in danger people generally lose their appetites.

Enormous life change: Beside biochemical changes, childbirth /motherhood changes every possible aspect of a woman's life--sleep patterns, relationships, work, lost freedom, changed relationship with partner, not to mention a universe of personal psychological meanings that "motherhood" may hold for a woman. For good info info, see http://www.Ncbi.Nlm.Nih.Gov/pubmedhealth/pmh0004481/#adam_007215.Disease.Causes.