I am writing a novel in which the protagonist meets a visitor from a "dimension" where mathematical realism is literally true. Namely, anything that the visitor can logically conceive of he can also "will" into existence.
Naturally at first the protagonist is pretty jealous of the visitor. After all, who wouldn't want to be able to summon a steaming hot burrito whenever they feel hungry. But now I need the visitor to convince the protagonist that his life isn't so bad after all.
So I am collecting a list of reasons why mathematical realism might be bad. For the sake of this article, assume that there is a society of "freely interacting" beings in the mathematical realism world, so you can only "wish" things into existence in my world if I allow you to. Or we could create a shared world where things can only be created according to a shared set of mutually agreed upon rules.
Here are some of the "bad" things I've come up with so far, but I'd like to collect more.
One obvious downside of being able to wish anything into existence is that you might (intentionally or not) wish something really awful into existence.
A favorite topic of rationalists. If you can wish for anything, there's a real chance of repeatedly wishing for things that gratify your short-term reward system. This results in either an endless loop of obsessively doing the same behaviors over and over again, or "burning out" and being unable to take pleasure from those types of rewards.
The real world is fascinating and filled with a seemingly endless set of things to explore or discover. Living in a world where anything can be created by thinking also means living in a world where the only things that exist are those that you can imagine. A sufficiently clever person might get around this by (for example) simulating the big-bang and recreating the entire universe. But someone who isn't sufficiently clever might just find themselves surrounded by the small set of things they can easily imagine, unable to ever experience surprise.
Lack of challenge
Similar to playing a game with all of the cheat-codes, living in a world where you can solve any problem simply by wishing might feel dull and unrewarding. Again, a sufficiently clever person can simply create restrictions and challenges for themselves, but the temptation to "cheat" will always be there.
Even if you are clever and virtuous enough to create a decent life for yourself, there's a chance that everyone that you know might be wireheading or too scared by the eldritch abominations they have created to be your friends.
If you're clever, you can simply "make" friends, but these relationships may feel shallow or unreal for the same reason that "cheating" takes away the sense of satisfaction of overcoming a challenge. If someone is literally created to be your friend, how can you ever know if the "really" like you or if you're just forcing them to be friendly.
Even if other beings cannot specifically create things in "your" world, there still may be other ways for them to harass you. Suppose there is some method that all beings use to communicate with one-another, they could span or troll you using this medium.
I hope to edit this post as I think of other ideas, but please post in the comments if you have any ideas I haven't mentioned.