Possible explanation why Wander was so afraid of the spooky forest but not afraid of places like Doomstone or people like Hater or Dominator:
Wander probably was scared those times – just look at his face when he first meets Hater, or how he shakes when he’s on Dom’s ship for the first time – but because these instances still involved people, his positivity and need to always give new people the benefit of the doubt and consider them a friend first instead of an enemy probably overpowered his fear, pushing his fear into the back of his mind. Plus, Wander seems to have very good instincts. While they may not have been correct with Dom, they were correct with both the Doomstone residents and Hater.
But with the spooky forest, there was no person – no possible new friend or new face to focus on – and the naturally spooky atmosphere plus the creepy poem and the lack of anything else to focus on probably caused Wander’s fear and instincts to go into overdrive, causing him to be almost uncharacteristically frightened rather than trying to stay positive and believe that there was something good in the forest worth finding.
Besides, it’s always sorta refreshing to see Wander show a sort of ‘negative’ emotion – and to show that despite knowing a ton and being a great teacher for those who need to get on the right path, there’s still one or two lessons he has to learn, or one or two instances where he’s the one who needs help.