When someone says something along the lines of "I don't understand why someone wouldn't take a shower"
The reason(s) seem obvious, they just don't care about their hygiene, they think it's a waste of time, so on and so forth.
The real thing they're trying to say is "I wouldn't do that personally" or "I don't like the fact that they're doing that" not "I don't understand why".
And when you reply with "well akchshually I understand why they're doing that" it seems like you're the type to not shower as well.