Mussolini's personal motivations in joining Germany in WW2 were entierly self serving, and not due to some ideological loyalty. Mussolini's goal was to expand Italy's power and influence through Europe and through colonies in Africa. Italy even had its own concept of Lebensraum, it was known as "spazio vitale" which means (like Lebensraum) "living space". Although the concept differed in that the lands Mussolini desired were considered, to some extent "Italian", where as the majority of the lands Hitler had no German and were to be made German through colonization. Mussolini wanted control over Nice, Corsica, Malta, The Dalmatian coast, Slovenia, Albania, and the Italian parts of Switzerland. He also sought control over Egypt and Sudan, as well as Tunis and the French and British parts of Somalia. Mussolini wanted unbridled control over the Mediterranean sea, and his foreign policy was geared towards achieving this. As for the territories that Mussolini didn't directly control? They would be used as buffer/puppet states; this was part of the reason Mussolini intervened in the Spanish civil war.

Mussolini and Hitler actually clashed in the early 1930's, almost exclusively over the issue of Austria. Hitler had been sizing up Austria for annexation since he got into power, and that clashed with Mussolini who saw Austria as a useful buffer state. Hitler eventually flew to meet Mussolini at the Venice Conference in 1934. I've written about that here. At this point Hitler admired Mussolini as the founder of fascism, but the feeling was not mutual and Mussolini despised Hitler as an unoriginal, buffoon. The conference was a disaster and Mussolini's hatred of Hitler grew even more. Their relations soured even more when Austrian Nazis murdered the Austrian chancellor, Dolfuss, who was a close friend of Mussolini's. When Hitler again moved for Anschluss with Austria, Mussolini threatened war and moved his troops into the area, which caused Hitler to back off. Because of this Mussolini was viewed as a natural counter balance to Hitler's Germany, and Mussolini played both sides to try and get a favourable arrangement. Mussolini in 1935, along with Britain and France, signed an agreement, in a small Italian town called Stresa, that guaranteed the independence of Austria and formed a common front against German aggression. This would form the "Stress Front" which would be officially eliminated in 1939 (although it became meaniningless long before that).

Now this all changed when Mussolini visited Berlin in September of 1937, there he witnessed a parade set up by the German army, and came away convinced that Hitler was ready for war. Mussolini also became more and more ideological in his diplomacy. Where as before he had no issues dealing with the democracies of Britain and France, he began to drift close to Hitler. Mussolini's aggressive actions in Ethiopia, Spain, and Albania and his claiming of French lands also turned Britain and France against him. Mussolini eventually signed the Pact of Steel with Hitler in May of 1939, putting Italy in the Axis camp. The pact entailed the following:

Article I Article 1 stated that Germany and Italy were to stay in communication with one another, in order to 'come to an understanding of all common interests or the European situation as a whole'.

Article II Article II obliged Italy and Germany to follow a similar foreign policy: for example, the two countries agreed, in the event of any 'international happenings', to enter into mutual consultation.

Article III Article III promised the full military support of the signatories should the other country go to war.

Article IV Article IV supported the intentions of Article III, encouraging the establishment of greater cooperation in 'the military sphere and the sphere of war economy'. This article also supported greater communication between Italy and Germany in order to achieve economic and military cooperation.

Article V Article V compelled Italy and Germany to agree to all future armistices, further supporting increasing military planning between the two countries.

Article VI Article VI of the Pact instilled the importance of maintaining relations with countries which were friendly towards either Italy or Germany.

Article VII Article VII of the Pact of Friendship and Alliance between Germany and Italy dealt with the validity of the Pact: it stated that the Pact came into force upon completion and that it would last until 1949.

So to sum it up, Mussolini's personal ambitions were to establish clear domination over the Mediterranean. As for the dream about creating a new Roman Empire, it was more of a rallying cry and a propaganda tactic to get popular support for the conquests Mussolini had in mind. Mussolini believed that the Kingdom of Italy was the direct successor of the Roman Empire and thus they had a duty to expand their borders and turn the Mediterranean into an Italian dominated sea. The idea of Italy being the New Roman Empire was pushed hard, and the populace generally did rally behind the various wars of Italy, with the exception of the war against Britain and France. There was some mild enthusiasm in the first year of WW2, but that quickly went away as Italy stumbled from defeat to defeat. As Italian morale died, so to did the idea of a new Roman Empire.

Mussolini by Dennis Mack Smith

The Fall of Mussolini: Italy, Italians, and the Second World War by Phillip Morgan

Italian Foreign Policy in the Interwar Years by H. James Burgwyn Mussolini Unleashed: 1939-1941 by Macgregor Knox