Big Little Lies knows the importance of finding BFFs. Although Jane, Madeline, and Celeste are very different people—with different interests, hobbies, careers, and backgrounds—they're bound together.
This has a bit to do with the fact that they're raising kids that are approximately the same age, a bit to do with the fact that they like talking trash about the Blond Bobs, and a bit because they're all just kind, funny, interesting women. But their friendship isn't portrayed as 100% roses and sunshine: they feel envy, frustration, and annoyance with each other. Sometimes they disagree.
But in the end, they have each other's backs. Because that's what friends are for.
Questions About Friendship
Why is Celeste jealous of Jane?
Does the ending of the novel suggest that the three main characters will become friends with Bonnie?
Why does Madeline start the "Erotic Book Club"?
How do the three central friendships change over the course of the novel?
Chew on This
Big Little Lies portrays friendship as being as bonding as marriage.
The friendships in Big Little Lies are formed almost exclusively by proximity; these women have nothing in common with each other.