Born of the flesh has the attachments inherited from our ancestors (Adam and Eve). To be born of the Spirit means to me that you have been 'called' by the Spirit to repentance and have answered that call. It somehow changes your 'DNA'.
Here is a hint at that from when God visited Ezekiel in the chariot.
19 And I will give them one heart, and I will put a new spirit within you; and I will take the stony heart out of their flesh, and will give them an heart of flesh:
20 That they may walk in my statutes, and keep mine ordinances, and do them: and they shall be my people, and I will be their God.
Yes, the 'attachments I spoke of are the sin nature. It's what happened after the disobedience in the garden.
It is described as 'being naked' and why God had to make atonement for them with the animals he skinned for their 'coats'. That has stayed in our DNA for all these millennia. That is why Jesus said ' You must be born again'. You need to get back to that 'clean' state Adam had at the beginning. Until that is accomplished, you remain in an unclean state. This is what being born of the Spirit means.