After reading Paradise Lost, why does the creature think he is like Adam in the book (as defined in Mary Shelley's Frankenstein)?
The creature, in Mary Shelley's Frankenstein, finds Paradise Lost and begins to read it. After reading the novel, the creature comes to have feelings regarding his parallel to Adam. The creature explains, in chapter 15, his understanding of the novel and his likeness to Adam.
It moved every feeling of wonder and awe that the picture of an omnipotent God warring with his creatures was capable of exciting. I often referred the several situations, as their similarity struck me, to my own. Like Adam, I was apparently united by no link to any other being in existence; but his state was far different from mine in every other respect.
Here, the creature recognizes that his only link to another (similar to Adam's sole connection to God, given God was his creator) is that Victor created him.
After this initial recognition is made, the creature also recognizes that he is very different from Adam. The creature goes on to describe how he is different from Adam.
I sickened as I read. ‘Hateful day when I received life!’ I exclaimed in agony. ‘Accursed creator! Why did you form a monster so hideous that even you turned from me in disgust?
Therefore, the creature's only parallel to Adam was that he is only linked to Victor (and no one else). It is this knowledge which allows the creature to make another connection: he is more like Satan than Adam. The creature, after reading Victor's journal regarding his creature's existence, finds that he has been alienated and exiled like Satan.
The creature believes he is like Adam in Paradise Lost because, as he says, "[he] was apparently united by no link to any other being in existence." In other words, like Adam was initially, the creature is the sole creation of his creator and has no one with whom to share his existence. He feels wretched and alone on earth.
However, the creature says that in every other way, he is dissimilar to Adam because Adam emerged happy and perfect from his creation, and Adam was carefully guarded by God (while the creature is not guarded in this way by Frankenstein). In addition, Adam was able to converse with beings greater than himself and learn from them, while the creature is not able to do so. Instead, the creature feels forlorn and lonely. The creature says that in most ways, he is more similar to the outcast Satan than he is to Adam. He asks Frankenstein to create a companion for him to make him feel less lonely and miserable, but Frankenstein refuses.
The creature relates to Adam because he feels like he, too, has been hated and shunned by his creator in spite of his attempts to be and do good. He feels like his creator has abandoned him in a world that hates him for being what his creator made him to be. He feels alone in the world and asks that a help-meet (partner) be created for him. When his creator denies him even that, it further angers the creature.