I think Italy did the only possible thing in a situation that seemed very favourable.

NEUTRAL ARGUMENT: The neutrality was possible actually, see the Netherlands, and maybe it would have resulted in a better economical situation for Italy (being able to treat with both sides) and more political stability. Which in hindsight might have avoided the red years and Mussolini's ascension. There was a huge neutrality front in Italy, but the pro-war one prevailed even though not really in a trasparent way. The Treaty of London was in fact kept secret from the Parliament and known only to members of the Salandra cabinet. There was a fear that staying neutral would have doomed Italy's possibility of annexing the missing territories in case of a German/Austrian victory (and at the first it looked like Germany could get a decisive win on France's border with Von Moltke at the First Battle of the Marne).

PRO-WAR: The nationalism, the Risorgimento's sentiment was not completely gone. The foreign policy was still the same since the unity, so unifying all the Italian territories, and many were still lacking (Trento, Trieste, Istria, Dalmatia, Fiume). And Austria-Hungary wasn't surely giving them away, the only way to get them was joining the Entente or staying militarily neutral and pressing the claims during peace treaties, if France won.

AGAINST WAR: Joining the Central Empires wasn't possible. Italy's navy was nothing against France and UK's, and Italy needed their resources, in fact imported lots of goods from their colonies. And since Austria-Hungary didn't really have a navy, Italy would have put herself in a naval war against UK and France alone. That would have been way worse than Caporetto.

The war resulted in more than a million Italian deaths. The result were just a bunch of territories, not even all the promised ones. Speaking as an Italian, it was surely not worth it. Obviously I'm writing this with the results in hindsight, so nothing that really could help them decide.

The Great War has left more scars than goods to Italy, there are just a few "good things" that have survived this century coming from the war. One was the patriotism born as unity against the common enemy (Austria). I'm not obviously saying animosity is good and hating Austria is ok, just that it may have helped unity between people a bit (it was still a newborn country in many ways).

At least it was not all a big loss militarily speaking, Vittorio Veneto is well remembered today (as is Caporetto) and the "Canzone del Piave" was used as provisional national Anthem during WWII and after the Monarchy/Republic referendum held on the 2nd June 1946. But entering a war totally unprepared on a military/naval front is very stupid.

But to remark it, the "Vittoria Mutilata" sentiment was very real. All those efforts, those hard decisions, those fallen brothers and the victory is not even triumphant. No wonder some far-right sentiments became very popular after the war..