Do you think that Shannon expected Greg to be single and sad forever?
I think that most of her resentment towards Caitlin is caused by the fact that she thought that she was getting her happily ever after with Dina, but when that went south she got frustrated. And she blames Greg for a lot of her unhappiness, so I guess that she thought that he deserved to be alone, so him getting in a relationship and having a child with someone else must have hurt her feelings.
Shannon is a bad person.