I was this episode on my local FOX affiliate the other night and I thought that the writers could've had Lisa be more sensitive to Ralph Wiggum's feelings. I just can't see her publically humiliating Ralph the way she did and on the Krusty show no less.
She felt sorry for Ralph, and gave him the "Choo Choo Choose Me" valentine card. She didn't do it to extend feelings to Ralph, she did it so he would get at least one card. But it was an empty gesture because it had nothing behind it.
That puts her in the situation where Ralph misreads her intent. He pursues her, she doesn't say anything, because she doesn't want to "hurt his feelings".
But she becomes resentful of it all, precisely because it is unwanted attention but she is not saying it's unwanted.
Finally, she reacts to it all, and we have the seen at the Krusty show.