It's an adage, like "the early bird gets the worm". That particular one has no real connection to birds or worms, but you can get the meaning, can't you?
In your gf's case, I agree with Mike. Maybe she just thought it looked cool and "Japanese-y". Pretty stupid to me, even if she DID know the meaning. Better to buy a T-shirt "don't let life get you down". At least when that's old and worn out, you can throw it away.