The education, the training, and a mentor who walked with him for 40 years; Joshua had the whole package on his resume. God had His eye and hand on Joshua for a long time. God knew that Joshua could do the job He had called him for, but He also knew Joshua needed encouragement. In Deuteronomy 1:38 and 3: 28 God tells Moses to encourage him. Moses in 3:21 does encourage Joshua and it continues in 31: 6, 7, and 23 by telling him to be strong and courageous. This encouraging continues in the Book of Joshua chapter 1; God encourages Joshua and so do the leaders of the tribes of Ruben, Gad, and Manasseh.
Joshua gave what he got in Joshua 10: 25 when he encourages the leaders of the people to be strong and do what God called them to do.
I guess the life lesson here is your leaders need to be encouraged. They give a lot when they are doing their job so speak into their life and encourage them.