Emotions always run high when a baby is born, and not always in a good way. There are few other things that a person can do to change their life so radically, and it can leave them with some thoughts and feelings that they weren't expecting. This is common with new moms and the mental health struggles that can come from the hormonal and circumstantial changes of being pregnant and giving birth, but it also applies to other family members as well.
For a postpartum mom, how the baby's father acts can be a crucial part of whether they feel supported with the baby. It's not exactly groundbreaking to say that sometimes, dads can behave in ways that place more stress on an already stressful time. Having children is a test of a relationship in more ways than one, and never more so than the first couple of months after birth.