To thank someone actually meant to owe a debt of response to the one whom you were giving thanks. The parable clearly defines the role of servants. Servants willingly do what is expected of them. Servants expect no recognition or thanks for merely fulfilling their obligations. It is their job to meet the needs of their masters, and knowing they have done so is payment enough.
Servants are loyal, devoted, faithful, and obedient.
In verse five, the apostles have asked Jesus to increase their faith. Jesus responds with a parable about a little faith being enough, and then this message about being a servant. The disciples are to live out what is expected of them, without seeking recognition or reward. Life in that manner does indeed increase faith. To live as Jesus asked his disciples to live, with no thought for themselves, was not for the easily discouraged.