system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why are babies sometimes so unwilling to eat, when it is fundamental for survival?
gpt
Failure to thrive is sometimes caused by things that aren't obvious: some babies have acid reflux, some aren't signaling they're hungry in ways the parents pick up on, some have other health issues. 

Essentially, if a baby appears healthy and isn't eating, something is going on - and it needs to be investigated.