Someone probably noticed this already, but its news to me so whatever! Haha. Anyways, listen to my analysis! The following is the first verse of The Fry Song, which was cut from the final version.
Daddy, why did you leave me?
You created me, so don't you want to see me?
Daddy, why did you make me..
If you're not gonna take me to get a burger and shake-y?
My cursed blood is your cursed blood,
C'mon back and be my bud.
I can count your visits on my finger tips,
Come back and give me presents from your bussiness trips.
Daddy why did you father me,
If all you wanna do is bother me?
Don't you know that I love you and I only wanna see more of you..
It was already pointed out that when Simon found Marcy she was crying and left traumatised, mouthing the word "Daddy". This makes it apparent that Hudson was selfish and left Marceline so he could be safe in The Nightosphere while he just abandoned her. This could also explain Marcy's hatred for the Nightosphere and that she doesn't just have anger towards her father over stupid french fries. It is confirmed they are blood relatives on the show, as well in the verse ("My curse blood is your cursed blood"). It is also implied that Marceline wanted to do things with her father he never gave her the chance to do ("Daddy, why did you make me if you're not gonna take me to get a burger and shake-y?"). He could have also told her (since she was so little) that he was leaving on bussiness trips to help her, also explaining the line she said to Hambo ("I'm only hurting you because I love you."). This also explains how she saw Simon as a father figure rather than Hudson, he hurt her and once she grew up she realized it. She didn't want to hold a grudge and tried to get along with him, but the smallest selfish thing made her hate him once again (Daddy, why did you eat my fries?).
So I think Simon and Marcy might have to do with Marceline's complete analysis of her life and her relationships with both Simon and Hudson. Do you agree with my theory, why or why not? I want feedback c;