For if by the transgression of the one the many died, much more did the grace of God and the gift by the grace of the one Man, Jesus Christ, abound to the many. For if by the transgression of the one, death reigned through the one, much more those who receive the abundance of grace and of the gift of righteousness will reign in life through the One, Jesus Christ. So then as through one transgression there resulted condemnation to all men, even so through one act of righteousness there resulted justification of life to all men. For as through the one man's disobedience the many were made sinners, even so through the obedience of the One the many will be made righteous. It's a deep subject, but one that should be well understood by all people.
Sin is the reason for physical death.
Although we do not share the guilt of Adams' sin, we suffer the consequences because in reality, we all sinned. I usually explain it in terms of probation. We were all on probation in the garden. Therefore, physical death was imposed as a consequence of sin. This helps us to see the serious nature of sin.