I recently saw that "Find my iPhone" was turned off on my phone. I am 100% sure that I have never disabled Find my iPhone, and I believe it should be enabled by default. Does anyone know of a particular reason why this may have been disabled?
In order to disable "Find My iPhone" you'll need to know both the AppleID password as well as any device pass code. It cannot really be done accidentally by just tapping once or by someone that lent your device for a minute to call someone.
There's also nothing in iOS that automatically disables "Find My iPhone" for you.
However, contrary to what you write, it is actually disabled by default. So most probably you have never enabled it.