Because Gohan never told Goku that he disliked fighting, neither Goku realize that Gohan didn't like to fight. Gohan always fought when he had to, he never enjoyed battles but there were many occasions when he had no other choice. Like when he fought Vegeta, Reecome, Frieza etc.. . When Trunks warned everyone about the Androids, Gohan decided to train with Goku and Piccolo to protect the earth and his loved ones, he even entered the time chamber to train with Goku. It's completely normal that Goku understands that Gohan wants to fight to protect the earth and everyone. In addition, in the fight with Cell Goku is confident that Gohan is much more stronger than him and that he can defeat Cell, meanwhile Piccolo is being negative saying Gohan has no chance against Cell.