Nicodemus, Pharisee, ruler of the Jews...
This event occured while Jesus was in Jerusalem, after He has cleared the temple. Nicodemus' question shows that he has been considering Jesus for quite some time, along with others. And this reflects a certain contemporary thought at the time: that the Jews were expecting the Messiah to come and deliver. Jesus seemed to be a suspected candidate, but the things He did did not square with a lot of their conceptions about what Messiah would be like. And there, they were tripped up in all their thinking, even as Pharisees.
Note how Jesus begins the conversation. It has nothing to do with Nicodemus' statement. But Jesus knows his heart even before his mouth spoke and started the conversation immediately on what is important for Pharisees to know: the idea of being born again, to be saved.
"The wind blows where it wishes..."
Somethings happen, we experience it, and we believe that it is there. The wind. And we do not question it. How is it that we cannot bring that same faith into spiritual matters? That is the point in Jesus' declaration here about being born in the spirit. To be born again means to be born in of water and the spirit. Baptism? This is linked with John the author's references to baptisms before and after this passage.
"We know and testify what We have seen..."
Simply this, the only way to really know, is to believe the testimony of the one to whom true knowledge is found. We can try to increase knowledge through observing for our own, to believe only when we see things or experience things. But how often, even though the truth is before our very eyes, we still refuse to believe, but choose to explain it away by other "theories" or worse, ignore it. So then, perhaps this is something to consider with respect to philosophy of knowledge, perhaps the only true knowledge can only be obtained by faith in another. And this Another is the ultimate Another, who has seen what is the truth. So we can rest assured in His testimony.
"For God so loved the world..."
The famous verse. The only hope we have. Note, how we cannot do anything to have everlasting life other than to believe. And how this all stems from one point only. God's love. Love that saves, but also, love for which the rejection of is unimagineable.
Jesus' pleads here are entirely logical and reasonable. He who does truth comes to the light. Why then do people still not believe in the One? Choosing other alternatives? If they truly do what is true? What is their thinking and answer to Christ's words here?