I personally believe a large part of this is how shitty lots of dads are. I have seen far too many fellow dads who did almost nothing to help with the baby after the baby was born. I know one case where the dad took out multiple weeks in parental leave and mostly spent it on playing video games instead of helping his wife with the baby. Of course that would ruin a relationship. If you were both healthy and had easy going lives then maybe you never really had to depend on each other before and this may be the first point where you actually NEED your partner. And if that partner then doesn't step up you of course would start feeling resentful.