In approaching this question, it's important to understand what kind of person Doon is. He's a very serious, studious young man, with a real taste for knowledge. As such, he's none too popular with his classmates, who tend to regard him as a bit of an oddball. In the City of Ember, when students graduate, they're assigned jobs. Given Doon's prodigious knowledge, in particular his knowledge of technology, one might think that he'd be given a job appropriate to his aptitude and skills. Unfortunately for him, that doesn't happen. Much to his disgust, Doon's given the job of messenger, which as the name implies involves delivering messengers to the people of the City of Ember.
Doon's incredibly annoyed at being given what he considers such a meaningless job. The city's whole infrastructure is in the process of collapsing; there are chronic shortages of just about everything; and the lights are always going out. If these problems aren't addressed then there won't be anyone left to deliver messages to. Doon rightly believes that his talents would be much better employed as a Pipeworks laborer. That's the job that Lina's been assigned, but she's none too pleased about it. In fact, she'd much rather be a messenger. So Doon and Lina make a trade and Doon goes off to the Pipeworks to start his new job.
His first day on the job, however, leaves something to be desired. Doon's intellectual curiosity extends to insects; he takes care of a moth larvae that's about to develop into a cocoon. While working at the Pipeworks, he also collects a variety of bugs, which he finds an endless source of fascination. Sadly for Doon his co-workers don't share his interests. They subject him to mockery, teasing him mercilessly about collecting bugs for his lunch. But Doon will have the last laugh. His curiosity about the natural world may not earn him the respect of his work colleagues, but it eventually proves crucial in helping him and Lina find a way out of the city.