The Celts also formed the Kingdom of Galatia in Modern Turkey despite resistance from successor states, conquered Macedon and sacked it's most holy sites and famously repeatedly sacked Rome. The Celts were an incredibly successful civilisation or cultural group. Why is a matter of some debate, some like Peter Berresford argue that Celtic expansion was due to the highly developed military system of the Celts who were able to produce weapons of a high quality (indeed the earliest and highest quality Iron work seems to have been pioneered by the Celts as seen in the La Tene swords) and that their armies had more members who had seen combat and had perhaps received some basic training. This notion originates from the custom of minor tribal conflicts and near ritualised skirmishing between sub tribes or family dynasties and also games and fairs based around activities that simulate combat (athletic contests, javelin throwing, hunts etc.) we also have record of Germanic Celts banning obesity as it made a man weak for combat.

In most conflicts of the early Celtic era the Celtic armies would be facing armies composed mainly of drafted soldiers who had never seen combat before (as for example the Greek polis hoplite armies) whereas the Celts opposite them would have been familiar with at least some level of combat which is rather helpful.

The Celts from Roman, Greek and Macedonian record also seem to have mastered psychological warfare, Celtic warriors are described as fearless, men in the prime of size and strength stripped naked and screaming challenges might be placed in positions of high visibility but perhaps the most terrifying thing about he Gauls as Livy repeatedly mentions is the sophistication and order of their armies, he notes several times the well formed lines and ability to manoeuvre of the Gallic army at the Battle of Allia and also notes that the Romans were shocked and caught unprepared by how fast the Celtic army had moved suggesting efficient and sophisticated marching and supply at least relative to contemporaries.

I'm curious as to what drove them to be so traveled, and why they were so effective at it during a time when only few other societies (like the vikings?) seemed comparably widespread?
Obviously they had large conflicts, but the Celts didn't seem to be trying to build a coherent political empire. Were they just fond of raiding and exploring?