Just like natural birth, the new birth is a miracle of God. One could be adopted and still be the same person, not undergoing any changes. New birth, however, gives rise to a new person, a new creation. Third, birth implies a more dramatic process of change than does adoption. Adoption is a legal proceeding, a formality.
Birth is a physical process, even a painful process.
Many are brought to their new birth through affliction. John in verse 13 gives us three negatives and one positive concerning the new birth. The three negatives were given by John so as to circumvent common misconceptions concerning the new birth, and the salvation that accompanies it. The family, race, ethnicity, country, in which you belong has no bearing on the whether you are born again. This statement specifically was given by John to repudiate the belief of many Jews at the time that they were saved from God's wrath simply because they were children of Abraham.