My guess is, he's being overly cautious because the past incident happened not because of some strong monster but a trap of some sort, an unknown variable if you will, so there's no way to know when and where such incident will happen again. I mean, you can prepare for the strong, but it's impossible to prepare for the unknown.So far, I'm liking this manga. But I find it a bit annoying how overly scared the protagonist is around the heroine. I get that he's traumatized by what happened in the past. Still, even in a "kids' dungeon," so to speak, he's acting way too cautious. Of course, now, conveniently, an above-average monster shows up to prove his over-the-top caution isn't over-the-top after all. But... I don't know... Let's see what the next chapters bring.