What is the lesson learned in A Gate at the Stairs?

A Gate at the Stairs was published in 2009 and written by Lorrie Moore. In this novel, the protagonist, Tassie Keltjin, learns about how to handle the emotional pain that comes with love, loss, grief, and separation as she enters young adulthood.
There are multiple instances in this novel in which Tassie must learn to navigate the experiences of emotionally bonding with someone, only to have them taken away from her in the most excruciating ways. Her boyfriend, with whom she became infatuated, eventually reveals that he had been lying to her and must escape the area on being suspected of terrorism. She becomes attached to the young Emmie, who is then taken back into foster care, and she also loses her beloved brother to the war in Afghanistan. These losses teach Tassie about the inevitability of loss as you grow older in an unpredictable world.
A Gate at the Stairs also touches on the nature of misconceptions. Edward and Sarah Brink appeared to Tassie as an odd but unproblematic couple, with a violent history she only became aware of after quite a bit of time. She also missed an important email from her brother, asking if he should join the military or not, and she overlooked the fact that her boyfriend claimed to be Brazilian, yet spoke Spanish and not Portuguese. This novel touches on the fragility of life and our relationships with others. Acknowledgment of these details may have saved Tassie from a great deal of grief, which in part is why she declines to visit Edward Brink at the end of the novel.