1) Animals form families for survival of both individuals and species. For some animals, hunting alone allows better chances of survival so they remain solitary, while others require parental care of offspring over a long period for babies to survive and ensure the species continues. 2) Historically, human families formed for survival reasons as well. Babies require long-term care and assistance, so parents stayed together to provide food and security for helpless young. 3) Even as humans gained intelligence, the reasons why monogamous relationships form over long-term are still debated by scientists, with nature and culture both potentially playing a role.