The falling metaphor (if it is that) is widespread: to fall asleep, fall into error, fall into conversation with, fall in line with, fall a-weeping.

Fall here means simply pass into a state. This sense might have been a gradual change from a different sense (I can't get to the OED to check), which is why I doubt whether it's a metaphor.

As to pain, I can't see anything metaphorical in the collocation of pain and love. It's purely literal, most of the time.

... or is that just me?