“Jesus is followed as “the Lamb of God,” the sin-remover of the world...The connection between verses 36 and 37 means that the reason that John the Baptist’s disciples left John and followed Jesus is because Jesus is the sin-remover. Verse 36: John said, “Behold the Lamb of God.” And so in verse 37: The two disciples “heard him say this, and they followed Jesus.” This means that discipleship is first and foremost the expressed need for a savior from our sins.
In other words following Jesus is first and foremost not heroic. I’m desperate, I’m a sinner, I need a Savior. I’m not like David’s mighty men following David - “we’re gonna protect him, if he wants water, we’ll get water, we will not let anything happen to our king.” That’s not the way you follow Jesus. You follow him the way sheep follow the shepherd—because you need to be protected. We need to have our sins forgiven. We are weak, and he is strong. We are foolish, and he is wise. We are hungry, and he is bread. We are thirsty, and he is living water.
The point of the connection between verses 36 and 37 is that following Jesus calls attention to his strength, not ours. His goodness, not ours. His wisdom, not ours. Jesus made this crystal clear, “Those who are well have no need of a physician, but those who are sick. I came not to call the righteous, but sinners” (Mark 2:17). The reason these two disciples of John left him and followed Jesus was because Jesus is the Lamb of God. They are sinners. And he is a sin-remover.”
– John Piper
http://www.desiringgod.org/ResourceLibrary/Sermons/ByDate/2008/3439_We_Have_Found_the_Messiah