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There are reasons, first, he was just ambushed by bandits, though he said it didn't affect him, it actually did, giving him paranoia and trust issues, thinking there's danger everywhere.

Second, the only time he ever heard about adventurer's guild before joining are from Mira, and Sebastian. Mira told the story of how a group of 8 young adults went on their first quest and only three survive (one lost their hand, one went insane, one became a depressed alcoholic), so this framed that it was normal for adventurers to die easily. Sebastian said his friend died on their first quest (which mc thought was a low level quest because no other info given other than it's their first quest). So he would expect that even a low level quest can be dangerous. 

Also Naomi said lots of first timer died too. And she wasn't helpful, so his paranoia acts up. Like why would someone need an adventurer to deliver a letter to their neighbors? Since they can drop by the guild to make the request, surely they have the time to deliver it themselves, especially since they would have to pay and wait for an adventurer to take the quest if they put in the request instead. And he didn't know yet that low level adventurer quest are mostly running an errand or just menial tasks. He thought adventurers quests would have a bit of danger and adventuring in it. So when he's offered a quest that sounds so simple, he thought there must be something behind it.

Third, while he indeed has familiarize himself with the city, he also knows that there's only one guard at a time, either Sebastian or Ann, who patrol two districts by themselves and left the gate unguarded at times so anyone can just waltz in and hide in the city. Plus he just moved from a countryside where a citizen can ask a guard to check if the road to a nearby ruin is safe (meaning they have spare guards), to a city with only one guard at a time, and had bandits operating in the forest nearby, the new city wouldn't feel like a safe place for a while. And he's still relatively weak at this point, and don't know how to control Scarlet's beast power. If he got ambushed, either he's done for or rampage throughout the city possibly killing innocents, so he rather choose the safe route and ask for backup. 

Though he will eventually address this issues and grows from it.