Dear 100 Hour Board,
In creators Jerry Siegel and Joe Shuster's original storyline, what was Lex Luthor's stated reason for why he hates Superman?
This is an interesting question, because comic book continuity is wonky, particularly for characters that have been around for almost eighty years. But the short answer is "none," because there was no direct hatred for Superman in the early Luthor stories done by Siegel and Shuster.
Lex Luthor first appeared in 1940 (in Action Comics #22). His earliest appearances were in comics written by Superman's creators, Jerry Siegel and Joe Shuster. In his early appearances he is a scheming egomaniac, but he doesn't particularly hate Superman. He just wants to get rich and have power, and Superman stops his schemes. There was no clear distinct animosity towards Superman, he just got in the way. Luthor didn't show up particularly often in this era, but he did appear a few times.
Also, Luthor in these early issues is just a a guy in a normal suit with a full head of red hair. In a newspaper strip from 1941 a different artist mistakenly drew Luthor as bald, the first version of the more iconic version of the character known today.
The character is basically forgotten about for more than a decade. However, in the 1960s, DC was publishing a Superboy comic book (about the adventures of an adolescent Clark Kent growing up in Smallville) that introduced a teenage Lex Luthor. This teenage Luthor was an amateur scientist and huge fan of Superboy. Luthor saves Superboy from Kryptonite, as thanks Superboy builds him a laboratory, later on in the lab there's a fire and Superboy comes and puts out the fire with his super breath. The wind from this knocks some stuff over which makes a gas which makes Luthor's hair fall out. Luthor blames Superboy for the loss of his lab and his hair and vows to one day be more famous than Superboy. This story was written by Jerry Siegel, but not drawn by Joe Shuster. It's often cited as the origin story for Lex Luthor, because it's where we get a clear motivation for the character and an origin for his hatred of Superman (because, you know, Superman made him a bald teenager).
In the 1960s and 1970s Luthor does show up more frequently as a mad scientist character, in the 1980s they reset continuity and reintroduce Luthor as an evil business tycoon. So, if you hear that Luthor's original reason for hating Superman is because Superman made him lose his hair, that's mostly true, because that happens in the first story offering an explanation for Luthor's obsession with being better than Superman.
-Humble Master c/o Ardilla Feroz