To give a basic explanation, Norman mercenaries were hired by the various factions in the (then divided) south of Italy - when they returned home, they brought with them news of the opportunities existent in the Mediterranean; and in the 11th and 12th centuries, various groups occupied parts of Sicily and the South of Italy, later to be unified under a Norman King with the approval of the Papacy (although Papal relations with the Sicilian Normans were often fraught). Much of Western Europe was in the 11th and 12th centuries relatively stable; Muslim dynasties in the south of Spain were reaching their zenith, France was relatively united into strong fiefdoms, nominally vassals to the King of France. The fact that the Mediterranean itself was not enormously difficult to cross (in comparison to its mouth getting to Sicily might take just a few more days) also aided these expeditions; but Norman incursions extended as far as east of Constantinople against the Byzantines, where they made significant progress (Bohemond, later Bohemond I of Antioch after the First Crusade was a participant) but were ultimately repulsed. Fundamentally, the Normans were an aggressive and adaptive society which proved particularly able to adapt to the various circumstances of eleventh-century Europe, enabling them to take advantage of instability in Southern Italy (and, more famously, in England).