The City of Ember
How we cite our quotes:
Once they were out the door, Lizzie said goodbye and scurried away, as if Lina's bad luck were a disease she might catch. (1.58)
Because clearly, you should treat your best friend as if their misfortunes might be contagious. We get that Assignment Day is a stressful time for kids (since the luck of the draw will determine the job they work for the next three years), but you'd think that a friend would want to comfort their buddy who'd just gotten some bad news.
When they were younger, Lina and Doon had been friends. Together they had explored the back alleys and dimly lit edges of the city. But in their fourth year of school, they had begun to grow apart. (2.7)
Sometimes friendships just kind of fade away. Sure, no one wants it to happen, but it totally does. In the case of Lina and Doon, a silly accident where she laughs at him when he feels humiliated is the cause for them splitting. Maybe you've been in a similar position before, where common sense loses out to pride, at the cost of friendship?
When the lunch break came, Arlin took her lunch sack from a pocket in her tool belt and went off to meet some friends a few tunnels away. (3.25)
Arlin isn't too friendly to Doon on his first day in the Pipeworks, but apparently she does have friends of her own among the Pipeworks laborers. It makes sense that coworkers would befriend each other, especially when they're in a dank and dangerous environment like the Pipeworks. The more people who have your back down there, the better.