Because you'll never know, so...

because you'll never know when you'll get knocked down, so you walk carefully.

because you'll never know when you'll contract a disease, be paralysed, be in a coma, or even die, so you should be cautious and live your life to the fullest.

because you'll never know when you'll strike the lottery, toto or wateva shit lucky draw, so you just buy them.

because you'll never know when you'll fall in love, so you just walk...

because......., so........,
that's why.