I was talking to a guy the other day and he relayed this to me, which I found very insightful:

When a baby is born, its mother bonds with it.  She often will remark that she fell in love with him/her the moment she held him/her.  The smell, the warmth, the trembling fragility of a new baby generates a deep maternal resonance in mothers.  For fathers it's different.  When he first holds his child.....  When he first sees his wife cradling his son/daughter.... He bonds too.  But it manifests itself differently.  He feels a powerful upwelling of protectiveness.  He thinks (often subconsciously), "I will KILL anyone who gets too close.  I will murder anyone who tries to harm them."  Men often remark that they feel "on-guard" in the room with their new mother and baby.

Men and women are both parents.  Both feel love and protectiveness.  But for each, the reactions to a new baby can be quite different.  I'm a step-parent myself so I can't speak from experience.  Maybe it's different for different parents.  But this guy's insight into parental bonding was very interesting.... anthropologically speaking.