Julius Caesar took official command of his provinces of Illyricum, Cisalpine Gaul and Transalpine Gaul in 59 BC. His original desire was likely to pursue glory against the further reaches of Illyricum and Dacia, but events in his new provinces soon changed the plan. In Gallia Narbonensis, the stretch of southern France connecting Spain to Italy, the Gallic people had largely been assimilated into Roman culture over the course of the last century.
Beyond this territory to the north was a vast land comprising modern France, called Gallia Comata (long-haired Gaul), where loose confederations of Celtic tribes maintained varying relationships with Rome. These Celtic tribes, while primitive compared to the standards of Rome, traded abundantly between themselves and the Roman frontiers. For the most part, a general peace reigned between the tribes and Rome for the better part of the last century, but external pressures from Germanic tribes started unsettling the relative calm.
The Romans, however, had a long memory and fear of Gallic invasions that led to the sacking of Rome in the early 4th century BC was ever present. Additional tribal migrations of the Germanic Cimbri and Teutons in the late 2nd century, though defeated by Caesar's uncle Gaius Marius, merely confirmed these fears. If the Romans had legitimate fear over potential problems with the Gauls, then they were terrified of the wild, uncivilized Germanics. The Celts too, according to Caesar, were apparently despised by the Germanics for their more refined (weak) culture, and also had reason for concern. While Caesar was Proconsul of Spain between 61 and 60 BC, there was already considerable unrest on the Gallic frontier. The Germanic leader Ariovistus had invaded Gaul and raided the border regions, but Caesar quelled the situation at that point by arranging an alliance with the Germans.
Early in his Gallic governorship, Caesar still misread the situation and had 3 of his 4 legions stationed in Illyricum, but he would soon come to the realization that the real danger and opportunity was in Gaul. Understanding the emotional link that the Roman people had with the people of this region, Caesar began to alter his objectives. He was also likely quite aware of the great trade routes that the Rhodanus (Rhone), Rhenus (Rhine) and Sequanus (Saone) rivers provided. These rivers very well could've provided the most important exterior trade routes in the Roman world. Vast raw materials could be shipped in from the North and North West, while a booming market in Roman luxury items was beginning to go the other way.
Gaul too, was a veritable gold mine in potential plunder. Caesar, though at times showing a moderate respect for Roman law, would need a viable excuse to advance north and avoid legal issues in Rome. Within a short time of his arrival, an excellent opportunity for military glory and to further strengthen his bond with the Roman people would present itself.