By the end of the novel Sula has died, most of the residents of the Bottom have died, and Nel finds herself alone.
Yeah. It's not exactly uplifting.
When she finally cries for the loss of her friendship with Sula, Nel opens up a lot of questions for us. We learn that her cry has "no bottom and it [has] no top, just circles and circles of sorrow" (1965.73). Does this mean that Nel can't find relief from her sorrow now that Sula is gone? Is she crying because she's angry at herself for not realizing that it was Sula and not Jude that she really missed?
And how does Nel's sadness for Sula change how we see Sula? After all, Nel is probably the one person who is most justified in her anger toward Sula, but she seems to let this go by the end of the novel. Are we supposed to do the same? Nel misses her friend, despite the fact that Sula stole her husband, but in the end, their friendship endures more than any other in the novel.
Maybe the ending challenges us to reconsider what friendship and forgiveness really mean. Maybe it challenges us to reconsider our allegiance to certain characters in the book and our distaste for others. The ending doesn't seem happy in the traditional sense, but Nel finally gets the release she's been needing for years. So while the bottomless cry does seem to open up questions, perhaps it provides a resolution of some sort for Nel.