I'm not extremely picky, but I'm definitely pickier than most people I know, so maybe I can provide you with an answer, or at least my account of it.

I'm picky whenever possible, in the sense that, if I have the option to choose, I'll choose what I know and like, over what I don't know. It's a pretty simple equation really: either you have what you know and enjoy a nice meal, or you have what you don't know and maybe enjoy a nice meal.
In the first case you're sure you'll enjoy your meal, in the second, you're not. You might enjoy it, but it's not a certainty. Sure I may not know until I try, but that's perfectly fine because I haven't lost anything by not trying it, I've simply enjoyed a nice meal.

If there is no choice, however, I'll eat whatever is being served without being picky, if there isn't anything to be done about it, there's not much use in making a fuss.

As for how someone can eat the same thing every day: while I wouldn't eat the same dinner every day (a balanced diet is important and there are plenty of meals I do like), I've had the same breakfast for years. Good food is good food, and there isn't much to be added, really. I don't see how you could grow tired of it, sure it may not taste as good the 1000th time as the first time, but it'll still be good food.

Anyway, those are just my experiences. As I mentioned I'm not extremely picky but hopefully I was able to offer at least some insight.