He must increase, but I must decrease.

 When John relates conversations that Jesus had with enquirers, he has a way of following a certain scheme. We see that scheme very clearly here. The enquirer says something ( John 3:2). Jesus answers in a saying that is hard to understand ( John 3:3). That saying is misunderstood by the enquirer ( John 3:4). Jesus answers with a saying that is even more difficult to understand ( John 3:5). And then there follows a discourse and an explanation. John uses this method in order that we may see men thinking things out for themselves and so that we may do the same. 