I can understand the confusion.  On the surface, it seems like Jesus is claiming not to be good, and not be God.  But actually, He's asking why the man is calling him good, when the man actually doesn't believe He is (reading on in Luke 18, you'll see that this man favors his own wealth over Jesus).  Essentially, Jesus is saying, "If you don't believe I'm God, then why are you addressing me in a term that ultimately applies to God alone?"

It's true that people can be 'good', but only in a relative sense.  Someone who is charitable and loving would be someone most of us would call 'good'.  However, in relation to God, none of us are good.  We're all sinners, and none of us are so good that we deserve to be with God.  It's only through the forgiveness of God through the sacrifice Jesus made on the cross that we are united with God.  Ultimately, only God is good, and (as Jesus is saying here) only those who recognize Jesus as God have the right to call Him good.