Mussolini wanted to build a new roman empire. He needed an ally against Great Britain and France.
Explanation:
Mussolini rose into power when Italy experience an economic recession. He sold the idea of returning Italy back to the glorious days when they're the ruler of Europe during the peak of Roman Empire. Many Italian were desperate for new leadership, so they believed in him and give him their support to reach high position within the government.
In order to achieve this ambition, Great Britain and France possess the greatest challenge for Mussolini. Those two countries were regarded as the strongest nations in Western Europe.
This was the reason why he's so eager to ally himself with Hitler. He need Germany's military resources and aid to be able to defeat Great Britain and France.
