Why are hydrogen ions always associated with another molecule?

I was reading N.C.E.R.T. class XI part 2 book and chapter Hydrogen,when I came across this statement:



> 
> Loss of the electron from hydrogen atom
>  results in nucleus ($\ce{H+}$) of ~1.510–3 pm size.
>  This is extremely small as compared to normal
>  atomic and ionic sizes of 50 to 200pm. As a
>  consequence, H+ does not exist freely and is
>  always associated with other atoms or
>  molecules.
> 
> 
> 


So a question arose in my mind that why it is a consequence of small size of hydrogen nucleus that $\ce{H+}$ does not exist freely and is always associated with other atoms and molecules?




The way I understand it is (and my understanding is by no means perfect, or complete), as you pointed out correctly: a hydrogen ion is in fact a proton. The proton is a "bare charge" and as you rightly said, "tiny". a
This makes it extremely reactive (in a sense), and thus in a chemical system of any sort would immediately seek out and associate with the electron clouds of a surrounding molecule. 


For instance, in an aqueous solution it forms $\ce{H\_3O+}$ (hydronium ion, which are further solvated by water molecules and form a manner of aggregates).


An extremely interesting way of looking at it was pointed out in the comments (just putting it here, so no one misses it)



> 
> An interesting way to thinking about the reactivity of a bare proton is to calculate its charge density and compare with other monocations, such as Li+. This will make it clear why free protons will abstract electrons from just about anything. – Nicolau Saker Neto
> 
> 
> 


Do note (as far as I know, someone might correct me if I am wrong) free protons are fairly stable (by free I mean not bound to electrons or other nucleons) and usually don't decay into other particles. They exist in several naturally occurring situations, high energy plasmas come to mind.


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