it's so perfect that he looks at his reflection to gas himself up because if he chose to turn, he wouldn't have a reflection anymore!

carlos says in the adventuring party that wetzel chose human life both because he's got drago as an example of life getting better even through grief, and that he realized with dracula's letter and having to defend the castle that there isn't really a point to being undead because you still have the same problems, you just also have them for an immortally long time (read: forever).

if he'd chosen to turn into a vampire, he would have eventually needed to gas himself up because ofc he'd still encounter the same problems in that castle working for dracula, he'd just have them undead, and then he would pull out his mirror and he wouldn't even see himself anymore--literally and figuratively. instead, that mirror was the starting point to gaining a different perspective to living a human life, and in the end, that's what he chose: to keep seeing his reflection, to keep being himself, untainted by undeath and opening himself to new experiences.