This collection deals almost entirely with characters who are immigrants or second-generation Americans, so instead of one narrative, we see an entire social class of Americans, participating in a similar narrative of loneliness, resistance and a fight for life in a world that doesn't go easy on them just because they're immigrants. But this is not a collection about immigrants; it's the story of people. Regular problems that happen to everyone, like death, and loneliness, and substance abuse—but with the added layers complexity caused by minority social status and the isolation that can be caused by that.
Look at the narrative arc of Part Two. Hema and Kaushik have every reason to be together. As children, they were good friends, but after the two families drifted apart, Kaushik's mother dies, and Hema and Kaushik drift apart before finding each other two decades later in Italy.
This story helps to paint a picture of the theme that ties all the stories together. Hema isn't sure about her love life, because somehow, she still feels alone in her relationship to Navin, and Kaushik is by far the more tempting offer, but before she can make up her mind to leave Navin, Kaushik dies. If Kaushik represents community for Hema, as "Once in a Lifetime" implies, then this is the story of how confusing and unfair loneliness can be for people who belong in minority communities in America. Life's normal wear and tear cause the problems associated with young life in America to become nearly unbearable for many characters in this collection.
But essentially, this is not the story of immigrants losing and failing. It's the story of the resilience of the human spirit, in spite of challenges, and it represents a challenge to the reader to invest in those around them, because when bad things happen, we can't deal with those problems alone, or we'll fall apart emotionally, regardless of our ethnic or religious background. Jhumpa Lahiri is painting all humans as a family by combining similar narratives from different people's points of view.