First off, Joshua was still a new leader. He was likely shaking in his rookie boots. He saw what kind of consequences Moses had for direct disobedience.
Second, Moses was Israel's ideal leader and he had just died. Moses was prophetic, priestly, and "knew YHWH face to face" like nobody else ever has [Dt 34.10-12]. Tough act to follow.
Third, YHWH told Moses in Dt 31.16-21 that the people weren't going to be the greatest snapshot of godliness in the days to come. They were going to prostitute the covenant love of YHWH. Needless to say, Joshua wasn't leading 1,000 saints.
Fourth, Josh's job was to conquer the promise land. This is easier said than done when those in the promise land not only out number you, but are sometimes huge Goliath-ish fellows.
An 18th century rabbi [the Vilna Gaon] maintained that "strong" referred to the body and that "courageous" referred to the emotions. Even if this is a false distinction, noticing the above four reasons why Joshua needed to hear this repeated proves that YHWH is holding out a holistic hope to Joshua.
He does the same to us in another Joshua.
Of course, Jesus was His Greek name. Both He and rookie-warrior Joshua would have gone by the Hebrew or Aramaic "Yeshua" [meaning "YHWH is salvation"]. This new Joshua grants present hope because He leads into a promise land where there is pure and eternal rest [Heb 4.1-11].