The catalyst for the story is Kate. She didn't raise a son, she raised a pet. He has to do and live everything she wants, not what he wants, and now that he's trying to do something for himself, she sees it as an offense and has gone crazy, manipulating and ruining his life because it's supposedly better for her. She's a psychopath!
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That's not entirely true. The catalyst is Johnny. His actions and lack of actions are what affect everything. Kate does everything she does with Dexter to stop Johnny's bullying. Because Johnny can't wipe his own ass (Though that is Kate's fault).
And it's not that Kate is mad Johnny has a will of his own. She's been more than supportive with his passion for art and his desire to pursue an art career. Even encouraging and reminding him about the goals he decided for himself. Her problem with Johnny pursuing Lesley is precisely because it's Lesley. She knows that Lesley's manipulating him. She knows what kind of person Lesley is. Yes, she has attachment and abandonment issues, but those don't have anything to do with Why she doesn't want him to pursue Lesley. Let's not forget that Johnny pursuing Lesley has been what pushes Kate's relationship with Dexter further. Because whenever Johnny progresses with Lesley, Kate has to do more to keep Dexter from hurting Johnny. Which makes Johnny the catalyst not Kate.
Kate has always been supportive with Johnny's choices. Except when it came to other girls. But I don't think we can just say that she hates the idea of Johnny getting a gf, since the whenever she opposes Johnny getting close to a girl, it's a girl she hates.
Kate hates Lesley and Jacqueline. Kate just might not want Johnny to get close to them because she knows they're both manipulating him for their own desires.