He wanted <PERSON> to resist the dark side to be a jedi, and he feels that <PERSON> isn't capable of resisting when <PERSON> or <PERSON> tempt him(remember, <PERSON> had just shown he wasn't even able to lift a x-wing with the force, meaning that he might be tempted by a chance to learn more easily)
[<PERSON> sees his X-Wing is about to sink into the bog] <PERSON>: Oh, no! We'll never get it out now! <PERSON>: So certain, are you? Always with you, what cannot be done. Hear you nothing that I say? <PERSON>: Master, moving stones around is one thing, but this is... totally different! <PERSON>: No! No different! Only different in your mind. You must unlearn what you have learned. <PERSON>: All right, I'll give it a try. <PERSON>: No! Try not. Do... or do not. There is no try. [<PERSON> tries to use the Force to levitate his X-Wing out of the bog, but fails in his attempt.] <PERSON>: I can't. It's too big. <PERSON>: Size matters not. Look at me. Judge me by my size, do you? Hmm? Hmm. 605
And well you should not. For my ally is the Force, and a powerful ally it is. Life creates it, makes it grow. Its energy surrounds us and binds us. Luminous beings are we, not this crude matter. You must feel the Force around you; here, between you, me, the tree, the rock, everywhere, yes. Even between the land and the ship. <PERSON>: You want the impossible. [sees <PERSON> use the Force to levitate the X-Wing out of the bog and gets flustered when he does it] I don't... I don't believe it! <PERSON>: That is why you fail.
Therefore, <PERSON> thought that <PERSON> shouldn't go. <PERSON> probably thought after he (<PERSON> himself that is) lost to <PERSON>. He probably then made the idea that to become a Jedi, someone needed to resist the dark side, or better yet defeat a sith..
<PERSON> says: "You must confront <PERSON>. Then, only then, a Jedi will you be."
Therefore, after <PERSON> comes back, <PERSON> sees he can handle a sith, therefor saying his training is complete.
Props to <PERSON> (the actual account person on this website) for suggesting the Resisting the dark side idea. Before it was all just defeating a sith. 605