system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does it seem like I have to change my deodorant brand every year or so to avoid body odor?
gpt
The smell that emanates from your armpits is largely caused by the bacteria that makes its home there.  Deodorants, in addition to smelling pleasant, discourage bacterial growth.

Over time bacteria may grow resistant to some extent if you always use the same type of deodorant.  Occasionally switching can help combat that problem.