פירוש על קידושין 43:8

The master must love the slave for him to be bored. This is one way in which the rabbis interpret the words “because it goes well for him with you.”
Of course, the slave must also love the master. That is simply the simple reading of the verses.

For the slave to be bored, both he and his master must be healthy. This is another restrictive interpretation that they put on “because he is well with you.”