How come some men get side effects from Propecia and some don't

[baldozer]

Some men notice a lot of side effects when taking propecia, such as ED, Insomnia and such, while others don't have any side effects. I was wondering how is that possible. Our bodies more or less function the same way, and DHT is there in the body for a reason. What is the scientific reason that side effects only happen to a few men.