If I'm making something easier for you, then there's the implication that I'm helping you. I'm on your side and I'm going to do what I can to make the hard thing easier.
If I'm going easier on you, or making things easier on you, then it implies that I have been making things hard, either as an adversary or, as in this case, someone who is teaching you through hard examples, and I'm seeing that you're having a hard time understanding or otherwise coping with the toughness of my treatment of you, and now I'm going to go easier on you.
We can be hard on ourselves if we feel guilty.
Other people can be hard on us if they don't like us, or if they're trying to get us to do something we don't want to.
The phrasing in your quote is a little bit awkward, but I'm assuming it's that way because the author was trying to make a joke... He knows the correct form of the sentence is to use the word, "for", but as a way of acknowledging the difficulty of the subject he included the word, "on."