Personally, I don't think it was referring to making sure they loved Harry (you can't force someone to love, right?) because of earlier when, like some said, he was living in a cupboard. I think it was just referring to the fact that his "last" (the letter) described the blood protection. And maybe it wasnt a threat, but more a conscience thing. Like, she would know what would happen to Harry had he left (or the Howler made her realise it was serious).

I don't think there was much more than an explanation in the letter. But, for all I know, there could be.