Caesar had a mountain of ruinous debts, a growing number of enemies in Rome, and little at his disposal aside from his governorship and the legions that came with it. Pompey Magnus feared he would be outshone and displaced by Caesar, just as he had outshone and displaced Sulla.

Caesar's war was certainly not a necessity, of course, but an illegal/unauthorized campaign, the aims of which were to fill Caesar's coffers and enhance his popularity with the people. However, I think he was compelled to action by his circumstances.

Over the course of Caesar's life, there are numerous examples of a desperate gamble that should have ended in disaster, but somehow, he just kept getting dealt a royal flush: the Cilician pirate incident, the Gallic wars and Alesia in particular, his invasion of Italy, the Battle of Pharsalus, the siege of Alexandria. The man's entire career was a string of calculated risks that came up in his favor, until they didn't -- i.e. the calculated risk of trusting his former enemies and showing them clemency.

In Caesar's mind, I believe the Gallic wars were more than simply a means to an end. They were a gambit for political (and literal) survival. This wanton slaughter was palatable to the people of Rome because of their deep-seeded hatred for the Gauls; in the Roman psyche, the Gauls were their most fearsome and ancient enemy, rivaled only by Carthage in terms of cultural animosity.