Mazer Rackham, a character from Ender's Game, says that 'there is no techer but the enemy' because in the end, Ender learned more from bug.gers (the enemy) than from the humans. The bug.gers (also known as Formics in this novel), are an ant-like alien species which wanted to colonize Earth, however, humans fought them off. Ender had so much to learn from the enemy, which is something he couldn't do from his fellow humans.
I do agree with him, to an extent. Although your friends and family can teach you so much, you will grow even more through hardships and your battles with the 'enemy.' If you lose against them, you will know what you did wrong and you will do better next time. If you win, you will obviously know the right strategy against them and grow thanks to that experience.