Jesus is addressing his disciples. Here's his spiel.
It's certain that there will be plenty of things to trip you up, but here's a big boo to the person who's responsible for those obstacles. He'll wish one of those huge millstones were tied around his neck as he's tossed in the ocean. After all, that's a better fate than causing "one of these little ones" (17:2) to fall. Who are the "little ones"?
Jesus warns his disciples to be on their guard. These instructions are for them.
They should rebuke another disciple for sinning against them, but they should forgive him if he's sorry.
Even if this happens seven times a day, the rule is still valid. That would be a lot of apologizing.
The apostles request that Jesus increase their faith.
Jesus responds that faith as small as a mustard seed is still capable of uprooting a fruit tree and re-planting it in the ocean.
Need an illustration? No problem.
Your servant returns from a day of plowing or shepherding. You're not going to reward him and ask him to sprawl out on the couch for dinner.
Right? You're going to order him to prepare your dinner. You're going to make him don his butler's suit, serve your food, and pour your drink. After that, he's allowed to take his own supper.
What's the lesson? You don't reward a servant for doing what he's ordered. It's his job.
Oh, and it's the same for all y'all.
Your attitude should be that you're worthless slaves for doing only what you've been ordered.