Millions/billions have lived, died and suffered through the centuries. I have yet to find a reasonable - credible - explanation why God allows this. The original sin explanation never convinced me - why does anybody have to pay for the sin/s of another?
The mystery does not stem from what suffering is, but from what love is. When one ponders the God-man suffering and giving His life for creatures He created from nothing, one must acknowledge how impossible it is for us to wrap our minds around so great a love. He IS God after all and we are not.
Fr. Vincent Serpa, O.P.