You are very insightful to figure out that infidelity is a common thread in all of the stories of Friend of My Youth! Let's begin here and say that seven out of the ten stories feature a spouse that is cheating! Alice Monro is often speaking from a woman's point of view (in that only one of the stories has a male narrator), and it is often the woman who is committing adultery. As a result, the woman (in many of the stories) ends up leaving her husband and children in order to move elsewhere. Let's look at some of these stories and find out how infidelity is such a curious draw for so many women in society.
Let's begin with the story called "Differently." In this story, we see a woman named Georgia considering the answers to lots of questions and how the answers may have made things turn out "differently." While middle-aged Georgia goes back to her old haunts, she visits Raymond. In the midst of the story we find out that Georgia and Raymond and their mates used to be quite a close knit collection of people. As my introduction suggests, Georgia cheats on her husband, dismantling that group. Georgia considers what might have happened if she had acted "differently" and didn't commit adultery. As a result, she left that house near her friends. Raymond's wife, now deceased, had also tried to make a move on Georgia's lover. Georgia considers how "differently" things would have been if she had forgiven Raymond's wife. Georgia "almost" comes to the conclusion that if people didn't consider themselves invincible, they would act differently, but she ends up admitting that's a "joke."
Next, let's look at “Oh, What Avails.” In this story, it is the sister (again, the woman of the story) that is guilty of infidelity. Interestingly enough, this story compares a brother and a sister (all Canadian) who have taken their lives in different directions. Contrasted with the sister is the brother who has chosen to live a very conventional life through marriage (marrying the girl next door), staying in the same town, budgeting to live within their means, and remaining truly faithful. The sister, however, moved to Ottawa as soon as she could. She had a faithful marriage, but is now having an affair in an attempt to abandon that marriage. This is another story about a woman who rejects the conventional traditions because her needs are greater than what society provides.
In the title story, “Friend of My Youth,” we have yet another interesting example of infidelity. This story is about a love triangle! Because it's told from a narrator's point of view not involved in the "action," its an interesting view of the infidelity. Flora is the narrator. She is the daughter of the participant: her mother. It is the mom involved in the love triangle. Flora, in the accounts of infidelity, longs to hear her mother's voice again.
In "Five Points," we see yet another character taking advantage of freedom sexually. Brenda is a married woman in Ontario who (you guessed it) is having an affair. Strangely enough, in the midst of the infidelity we find some really neat details about a sweet shop in Victoria from Neil (Brenda's lover). Brenda finds at least some release in her affair, but she is disappointed. Brenda finds out that adultery is quite dull, boring, and even demanding... just like marriage is.
Finally, we have the story “Oranges and Apples,” which has a male narrator (finally)! This one is interesting because we learn about infidelity from a male point of view. The main character here is Murray Ziegler who is married and is worried about his wife's relationship with a good friend of Murray's. In the end, this friend does threaten their marriage which, of course, goes back to Monro's focus on infidelity which seems to be fated to all of these couples.
Thus, we can say as readers that Alice Monro is not afraid to approach the subject of infidelity as she tries to figure out what makes people the "happiest" in life. She focuses less on faith than she does on fate.
People are curious. A few people are. ... They will put things together, knowing all along that they may be mistaken. You see them going around with notebooks, scraping the dirt off gravestones, reading microfilm, just in the hope of seeing this trickle in time, making a connection, rescuing one thing from the rubbish.
In the end, we can see that Monro feels that having their curiosity satiated is what makes people the happiest.