English to Arabic Other Other Idiom

Hello,

This is a conversation between the writer and her sister about a guy she (the writer) seems to like but is kind of hesitant to go out with. So the sister tells her jokingly: Why don't you want to be happy?


[She answered me lightheartedly, but she was joking on
the square: “Why don’t you want to be happy?”

You’re probably wondering what this last one has to do with
my ego. I mention it because that line, Why don’t you want to be
happy?, slices to the very heart of how the ego functions.]

What does this expression mean? "Slices to the very heart of how the ego functions"?

Does it mean: to dive into the depths of "how the ego functions"?

Thanks!