Why are fluoride ions (and ions in general) reactive? Please bear with me—-I have a pretty meager background in chemistry but am trying to learn some! 

My question is about ion reactivity—-specifically of the reactivity of $\ce{F-}$. 
I often hear that the main reason the “weak” acid $\ce{HF}$ is so dangerous is because of the fluoride ion rather than the $\ce{H+}$. The impression I’ve gotten is that fluoride is the real hit man in this situation, with the $\ce{H+}$ only really serving to corrode barriers to entry. 

A search for “fluoride ion reactive” yields the following sentence: (source) 


  Unlike other hydrogen halide acids, the anion (fluoride ion) is quite reactive, and can form fairly insoluble salts with alkaline earth metals such as calcium and magnesium.


However, I think I’m fundamentally failing to grasp something. Why is this insoluble salt effect specific to fluoride? It’s obvious why fluorine is reactive, but why is the octet-satisfied fluoride reactive as well? And how does fluoride reactivity compare with the other halides? 

I have also heard people say that fluoride is a bad nucleophile, which makes sense given how tightly it holds on to its electrons. So I can’t quite wrap my brain around why fluoride would want to react with anything at all. Wouldn’t that just force it to share its extra electron with another atom? (Is there such a thing as “electronegativity” but for ions rather than atoms?)

After thinking about it, I realized my confusion extends to all ions in general. Why would “fully dressed” ions with full octets do anything but sit around on the couch eating Cheetoes?

There’s definitely something stupidly fundamental that I’m missing — hopefully someone can point me in the right direction!
Fluoride activity, or more properly danger, comes from the fact that formation of some salts of fluoride is energetically highly favorable for aforementioned metals. 

Calcium for instance, rather than in calcium carbonate, is more stable in $\ce{CaF2}$ form. Then, consequently, it will etch your bones eventually if there is a dermal contact with $\ce{HF}$. Other acids like sulfuric acid make a superficial effect and with a copious amount of water you mostly get rid of it. $\ce{HF},$ on the contrary, is a weak acid and slowly seeps through your epidermis to your bone.

$\ce{HF}$ is also reactive with glass, through silicon. It may also etch the beaker or any other container through time so you don't want to store $\ce{HF}$ for a long time in glass containers, actually same with bases to a lesser degree. 

$\ce{F}$ can even react with noble gases, you should just look at the last product and compare the overall stabilities of the current form and the final form. Fluoride simply more stable in $\ce{CaF2}$ solid than in solution.