@effingreally,
Hello,
You have asked an excellent question. There are, indeed, rules. I suggest that you check the Web for the use of "more" vs. "-er."
In a short post such as this, I can give you some advice from two experts. They write: "[S]tudents should understand that the basic form of the comparative is 'more' and that the -er forms are surface lexical manifestations of 'more' + adjective / adverb."
The experts also say: "A good rule of thumb for nonnative speakers is the following: when the adjective has two or more syllables and you are in doubt, use 'more'; the worst mistake you will make is a register error." (By "register," they mean formal language vs. informal, spoken language.)
Finally, one last thing. They remind us that sometimes native speakers use the -er form to get attention. They cite Alice in Wonderland when Alice says "curiouser and curiouser." Usually, we do not use -er with an adjective that ends in -ous.
James
The two experts are Mesdames Marianne Celce-Murcia and Diane Larsen-Freeman in The Grammar Book.