If we want to insist upon what we assert as true, then, reasonably, we must show true.
If we cannot, then we can do no other to put the assertion(s) down as either opinion or induction, at best.
There's only two ways to (dis)prove something: by (empirical) evidence and (deductive) argument.
Sufficient empirical evidence is often quite hard, if not impossible, to come by due to the nature of the means we are using here to communicate; a forum, basically letters behind a screen.
Leaves us mainly with deductive argument.
A deductive argument that is valid and sound always produces a true conclusion. Such an argument is considered to be proof - due to its intrinsic (axiomatic) nature - since it cannot (logically) be refuted.