Targum Yonasan says that when Moshe saw what an “Anav” Yehoshua was, he decided to single him out, since he was afraid that such a humble person would be persuaded by the spies. In Parshas Vayeilech I counted 14 times that it says: חֲז קְּוֶאֱמ ץ be strong and brave. We can suggest that maybe Yehoshua needed extra Chizuk, like Targum Yonasan says about his needing Chizuk against the Meraglim. Of course, we have no understanding of Yehoshua. But in our world, we understand that people who are by-nature overly humble need extra Chizuk.
The Arizal was M’chazek his Talmid R’ Chaim Vital, constantly telling him how great he was, etc. and even after his passing he appeared to him in a dream to continue being M’chazek him!
Somebody asked R' Gershon Liebman an Eitza for a person who isn't so good at learning Mussar. R' Gershon said, that just like by food, if you keep stuffing you develop a taste; when you keep stuffing him with more and more Mussar, he develops a good taste for it, if it's done properly. A person may have a very weak disposition, but if you keep giving more and more Chizuk, the person actually changes. You have no idea how people can have radical changes, if he is: הָעַקְ שָׁן יַצְלִּיחַ a stubborn one, who will be Matzliach!
The success of a person depends on how much Chizuk he has
