There is a curious progression in this passage. Christians sometimes don't see the connections, but they are there.
This is an often-quoted passage. Not so often noted, however, are the steps of forgiveness. After the sin, comes the rebuke. Most of us don't have the faith and courage to do this—and Jesus knows this. We just "forgive and forget" - with the result that our brother has no motivation to reform. We must rebuke. Then repentance can come. If we will not rebuke, we have failed our brother, and have refused to follow the command of our Lord.
Note, too, that repentance must be had. The object is to restore the brother to a right relationship to God and the church. If there is no repentance, this relationship cannot be restored.
The disciples' reaction
It seems disconnected, somehow, for the disciples to ask Jesus to increase their faith. It's as if Luke had mixed up his note cards in preparing this section. But this is not so. The disciples clearly understood the problem. The brother offends; I rebuke—and what if he gets angry and doesn't repent? Then what a fix I'm in! He'll tell me I'm judgmental; he'll tell me I'm unforgiving and not a very good Christian; he'll tell me I didn't understand that he had a right to do that. To step out against that human reaction is not normal; it is an act of faith. Jesus tells you to do it; you must step out on faith—rebuke, see repentance and forgive. It is risky business, that.
Our Lord makes clear that the starting point for this is just that: a starting point. Most of us won't do this the first time. But if we do—and do it in faith—God will increase that faith, just as naturally as the tiny seed grows into this large plant. But there is a danger in that, too.
Who owes what?
There is a temptation here to forgive (often without rebuke or repentance) and then tell God that (having forgiven) He is now in our debt. We feel that forgiveness racks up celestial brownie points. It is not so.
Christ reminds us that rebuke and forgiveness are the obligations of a disciple. They are not optional, for our own forgiveness depends upon them. It is a requirement, not an option. We are saved by grace, not by our forgiving others.