Picollo needs a weapon to fight the coming Saiyans. Gohan is that weapon, as he saw in the fight against Garlic Jr and then Raditz. Gohan is powerful, but childish and naive, with no skill as a fighter. Goku's willingness to die to kill Raditz was shocking for Piccollo, when he digests what happened. read more
Because earth has some really weak warriors. Seriously though the humans in dragon ball are weak as hell relative to the other species. In dragon ball every other species we see has widespread knowledge of ki and energy manipulation. read more
This almost father-like affection towards Gohan was definitely evident when Piccolo died whilst protecting Gohan from an attack from Nappa. Gohan and Piccolo developed a sort of father-son bond over the series, and so Piccolo will probably never refuse to train Gohan. read more