In the chapter titled "1940," Nel asks Sula why she slept with Jude three years prior.  Sula clearly tells Nel that she did not love Jude, nor was she really interested in him--he simply filled a space in her life.  She also tells Nel that love is risky, so loving Jude was out of the question.  Nel is shocked and upset by this response because Sula suggests that the friendship between the two women was ruined for nothing.  Sula, however, still does not understand why Nel is so angry with her for sleeping with Jude.  From Sula's perspective, Nel and Jude did not have an interesting relationship, and Sula was always rather free around men.  It almost seems as if Sula thinks that Nel as her best friend should be willing and wanting to share absolutely everything with her.