The Italians could see from a neutral standpoint how the war went in 1914 and 1915. The millions of casualties and the stalemate that developed especially on the Western Front.
And then, in 1915, at a point when I assume it was very obvious that it would be a long war and that there could be no quick victory on the Western Front, the Italians enter the war! The results are very predictable, and they ruin Italy.
Why did Italy expect anything positive to come out of joining the war? Wasn't it obvious that a quick victory over Austria-Hungary would be completely impossible? Why could they possibly want to join such a destructive war and not count themselves lucky for staying out of it?
Again, I am not an historian and I can only provide you with what I have learned.
How do I ever begin? Well... historically speaking, Italy had fought no less than four (4) Wars of Independence against the same enemy: the Austrian Empire, which would later become Austria-Hungary. It is no surprise then, that the Italians were mostly hostile to any kind of alliance with the very country they considered to be a sworn enemy.
Yet, in 1882, an alliance with France was out of the question because of a colonial dispute over Tunisia.
Due to this, the then Prime Minister Francesco Crispi saw fit to rekind the country's ties with A-H and signed a defensive pact - the Triple Alliance - with Austria and the German Empire, a pact deeply unpopular with both the populace and the intellectuals (can you see the parallel with WW2?). At that time, Austria-Hungary still held those Italian-speaking bits - namely Trentino, Trieste, Istria and Dalmatia - which were considered terre irredente, that is, territories under foreign domination where ethnic Italians were living and being oppressed. Moreover, that same year the execution of an Italian patriot by the name of Guglielmo Oberdan strained such relationship from its very beginning, and Crispi's dreams of getting back these lands by means of peaceful talks was shattered; it was clear that Austria wouldn't have ceded.
Which brings us to 1915.
Crispi, the main artifex of the alliance, was no longer alive or in power;
The new war had been initiated by Austria, so the provisions of the defensive treaty did not apply;
Emperor Franz-Joseph had in the meanwhile put in place discriminatory practices and restrictions against the ethnic Italians living in the Empire, further angering the Italian public opinion;
The nationalists and the intellectuals, such as Gabriele d'Annunzio, began stoking the flames of war so much that those in favour of war became as many as those who favoured neutrality (Catholics and Socialists).
This led to the signing of the secret Pact of London in April 1915, a move that Francesco Giolitti - the then pro-neutrality Prime Minister - did not agree to.
Giolitti tried to impede, or at least delay, the country's entry into war but with no success as the King too was now all for it; he then resigned in favour of Antonio Salandra - the very same man who had signed the Pact of London - who wasted no time (as the anti-war party was growing) to summon the Parliament and declare war against Italy's former 'allies'. In the end war-mongering, revanchism and rampant nationalism is what brought us into the war.
[...] Why did Italy expect anything positive to come out of joining the war? Wasn't it obvious that a quick victory over Austria-Hungary would be completely impossible?
Nobody expected a quick victory but many - too many, actually - were deeply unsatisfied with the then-current state of things and that was reason enough to go to war and be forever done with the Austrians. The war also provided a handy release for the nationalist tensions which had been boiling within Italy and Europe as a whole.
And actually, most felt that the lands they were promised were worthy a lenghty war so here you go.