Not always can we understand the meaning of Scripture from a few verses or even from the contents of a single chapter, Some times we need to search many scriptures, in order to understand.
In this case we are going to have to consider many other things, The first thing we have to consider is in what situation Jesus made the statements;
Let's take a look at the whole situation, this conversation began with Jesus consorting with what was considered the dregs of society in that era.
Luke 15:1 and 2 KJV
1 Then drew near unto him all the publicans and sinners for to hear him.
2 And the Pharisees and scribes murmured, saying, This man receiveth sinners, and eateth with them.
In chapter 16 we find Jesus is talking about The unfaithful servant, and after that he is confronted by the Pharisees.
Luke 16:14 KJV
And the Pharisees also, who were covetous, heard all these things: and they derided him.
There is a key to the rest of this exchange in the next verse:
Luke 16:15 KJV
And he said unto them, Ye are they which justify yourselves before men; but God knoweth your hearts: for that which is highly esteemed among men is abomination in the sight of God.
So what does that have to do with his answer to his disciples in chapter 17? Let's compare the rest of Chapter 17. Since the Pharisees were regarded by the public as being very righteous and one of the two parts of the ruling Sanhedrin.
Jesus had just gotten through giving them four lessons in the form of parables, on having humility, and telling the Pharisees that God did not approve of their seeking the accolades of men and not giving their worship the proper reverence.
And in verses 1 through 4 he had just given them a lecture about how they should show others the same grace that God showed to them.
Verse 5 is quite telling in that it shows that they were confused. The Pharisees spent most of their day praying and doing the work of the Temple. Certainly what they were asking Lord make us able to have more faith than the Pharisees.
Luke 17:5 KJV
And the apostles said unto the Lord, Increase our faith.
In verse 6 he is telling them that the power of faith is stronger than the roots holding a sycamore tree in the ground, and that if they had a faith that even though it started out small it should grow until it was thousands of times greater.
Luke 17:6 KJV
And the Lord said, If ye had faith as a grain of mustard seed, ye might say unto this sycamine tree, Be thou plucked up by the root, and be thou planted in the sea; and it should obey you.
In verses 7 through 10 Jesus is saying they expect their servants to take care of their master's needs before they take care of their own needs, so how unreasonable is it of God to expect that they do his will before the take care of their own. and not only that, but since they think that that is the way their servants should act, why not think they owe god the same duty.
Luke 17:7 through 10 KJV
7 But which of you, having a servant plowing or feeding cattle, will say unto him by and by, when he is come from the field, Go and sit down to meat?
8 And will not rather say unto him, Make ready wherewith I may sup, and gird thyself, and serve me, till I have eaten and drunken; and afterward thou shalt eat and drink?
9 Doth he thank that servant because he did the things that were commanded him? I trow not.
10 So likewise ye, when ye shall have done all those things which are commanded you, say, We are unprofitable servants: we have done that which was our duty to do.