I'm a game designer. Something can seem super fun in your head, but until it gets in front of you and you can play with it, there's no guarantee it's actually fun. And almost always, there was one or more flaws in your thinking that will have to be ironed out by playing it over and over again, preferably along with other people and getting their opinions on it as well.
I can't begin to tell you how many times I come up with this clever idea, mock up a bunch of components or start coding a prototype, and go... "oh, damn, this isn't much fun." I've been able to whip a lot of those failed ideas into something fun, but it often takes a lot of experiments with different ideas or borrowing pieces of designs from other games that have worked, a lot of time and rumination on the design (I get a lot of my ideas for new things to try while walking my puppy, commuting in a quiet car, or in the shower), and putting it aside to work on something else and letting my subconscious or things I encounter in life to inspire potential solutions to the problem (and often it'll still take multiple attempts to fix things).
The ones that drive me the most crazy are the ones where you try it for the first time and go 'This isn't that fun with this set of rules, but it's close to fun. If I can just find the right tweak, this can be great.' It can sometimes seem like it takes forever to find that right tweak. I've got some prototypes that I think are almost there, but I've had to put aside because I spent so much time on it and still couldn't figure out the right tweak for them.
There have been attempts to nail down Game Design principles. There are quite a few books you can get on Amazon that have some good insights in them, GDC lectures, Gamasutra articles, academic journals, etc. There's nothing that's totally universal, but the more you learn, the larger your design palette to draw from.