Italy had an alliance with Austria and Hungary it didn't decide to go to war with the Entente because it viewed the alliance as defensive, and Austria Hungary invading Serbia sparked the war. Just like Romania's later disastrous entrance into the war, the decision to come into the side of the Entente was after a protracted series of courtings on both sides. The allied arguments eventually won out as they promised significant parts of Austria to Italy as you said.

When Italy entered the war (May 1915), the war was already in full swing. The Western front had stagnated to brutal trench warfare for the Germans, and the Austrians were already fully engaged fighting in Serbia and more importantly in Russia. Italy entered the war and focused on immediately capturing territories along North of Venice (areas that it considered rightfully Italian). The Austrians, already engaged in fighting Russia to the East and Serbia in the Balkans, could not hope to go take an offensive grand strategy and for most of the war engaged in defensive patterns.

This is very generalized (3 years of fighting took place along the Italian front). But, for most of the fighting, the Austrians, thanks superior defensive terrain, the Italian army being deficient in arms and technology, and perhaps Italian incompetence, were able to keep the Italians from pushing into their territory and sometimes even go on the offensive. It is hard to overestimate the terrain that the Italian front was mostly fought on (alpine mountain tops and rough river valleys), and some of the highest altitude battles to take place took place on the Italian front. World War 1 was already a war in which we saw a vast disparity in offensive/ defensive capabilities for most of the war (eventually broken with advances in tanks and combined arms). For the Italians, they were facing the same kind of defensive strategies being employed as in the Western front, but adding to that in an alpine environment.

The Italians suffered by far more defeats than victories fighting the Austrians, but with pressure on the greater Macedonian front, Romanian entrance into the war, and the Brusilov offense the Austrians, despite continually successfully defending their lines, were in no position to try a general offensive strategy. A small window of opportunity opened up with the collapse of Russia and a redeployment of German and more Austrians to the front. At this time Italy had been fighting with little success for two and a half years of no success (May 15 to the end of 1917). The Italian army suffered from a huge lack of morale (much like the French mutinies in 1917) and this was seen as a possible time for a Central Power Victory. Fortunately for Italy, the entrance of America into the war bolstered allied hopes, and the immediately were able to help the Italian front with much needed supplies (but more importantly), draw any German participation away from the Italian front in preperation of their last great 1918 offensive. The Austrian string of luck ran out after 1917 and the Italians kept pushing with some success until (with the German war effort collapsing as well) an exhausted Austria collapsed at the end of 1918.

As for the European deals, you'll probably want to take a much greater look at the specifics of the meeting at Versailles.

what spurred Italy to change sides?

Italy feared a possible victory from Austria, and Austria violated the 7th point of the Triple Alliance by starting war in the Balkans.
Interventism was a widely discussed argument in Italy, and at the start of war Italy was divided in:

Interventists:
conservatives, nationalists and monarchists all wanted a stronger country and army, irridentists and republicans saw WWI as another Italian war of indipendence from Austrian rule; some leftist revolutionaries, that hoped the war would have favoured a proletarian revolution; futurists that saw war as the "hygiene of the world", and exalted his beauty and finally industrials that hoped to make money out of it.

Neutralists: catholics, most socialists and anarchists saw the war as an imperialistic conflicts, moderate leftists that hoped to gain Trentino from Austria in exchange for neutrality (when Austria refused, most had a change of mind. Differently from other interventists however, they knew the war was going to be a bloodbath, but saw it as unavoidable).

While Giovanni Giolitti supported neutrality, Salandra and Sonnino, supported intervention on the side of the Allies, and secured Italy's entrance into the war despite the opposition of the majority in parliament. On 3 May 1915, Italy officially revoked the Triple Alliance. In the following days Giolitti and the neutralist majority of the Parliament opposed declaring war, while nationalist crowds demonstrated in public areas for entering the war. On 13 May 1915, Salandra offered his resignation, but Giolitti, fearful of nationalist disorder that might break into open rebellion, declined to succeed him as prime minister and Salandra's resignation was not accepted. https://books.google.it/books?id=LFnIj7iyVQYC&pg=PA221&redir_esc=y#v=onepage&q&f=false

It should be noted that the interventionist side was a minority, but a very vocal (and violent) one, and was backed by France and UK and influential intellectuals like Mussolini and D'Annunzio.

Why did the other European powers renege on their deal with Italy and refuse them the land they had claims on at the end of the war?

Wilson's Fourteen points stated that "IX. A re-adjustment of the frontiers of Italy should be effected along clearly recognizable lines of nationality. " and that "I. Open covenants of peace, openly arrived at, after which there shall be no private international understandings of any kind but diplomacy shall proceed always frankly and in the public view. " (which refused international agreements made in secret, like the ones made between Italy and UK), and France feared for an Italian control of Dalmatia. https://books.google.it/books?id=VSx37W6OtOUC&pg=PT329&lpg=PT329&dq=The+Italians+must+somehow+be+mollified,&source=bl&ots=nQKmt_0sNO&sig=nx68IzB4hKuKPJsE7UYCtYCQtHM&hl=en&sa=X&ved=0ahUKEwi_mKumzObJAhUFvRQKHXLoC5YQ6AEILTAD#v=onepage&q&f=false