Short answers, pulled from the story.
Friendship is a relationship of mutual affection between people that functions as an invisible architecture supporting human society. It is a voluntary choice requiring constant maintenance and is a stronger form of interpersonal bond than an acquaintance or classmate relationship.
Children develop friendships through shared activities, physical proximity, and shared expectations, with 75% of preschool children having at least one friend. This figure rose to 78% through the fifth grade, and 55% had a mutual best friend, while about 15% of children were found to be chronically friendless.
Evolutionary approaches include the theory of Reciprocal Altruism which explains why individuals make friends with un-related others to exchange benefits. The Alliance Hypothesis argues that the function of friendships is to acquire alliances for future conflicts or disputes.
Strong social supports improve a person's prospects for good health and longevity, while loneliness and a lack of social supports are linked to an increased risk of heart disease, viral infections, and cancer. Researchers termed friendship networks a behavioral vaccine that boosts both physical and mental health.
Having more close friends is correlated with improved mental health and cognitive ability, but this association stops once around five friends is reached. After five friends, having more friends is no longer linked to better mental health and is correlated with lower cognition.