Friendship is the central force in Sula. It sustains the characters and gets them through some of the most difficult times in their lives. But friendship is also complicated and difficult. It causes pain and heartache as friends develop different understandings of what friendship actually means. Adult friendship can be decidedly more complicated than childhood friendship, as we see in the novel.
In Sula, friendless characters are better off than those who have friends, since their lives aren't as complicated.
Friendship sustains Sula and Nel, despite the momentary break in their relationship. Without each other's friendship to count on, neither woman is as happy and secure as when they do have each other.