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As was mentioned in the comments, the only actual personal documentation we have of this is the very account of Caesar. So if he didn't answer the question about their motivations, technically there's no answer to be had.
However, we do have ample evidence that this was not abnormal behavior on the part of Germanic peoples.
We believe that until roughly 750 BC the proto-Germanic culture was resident only in Scandanavia (including Denmark), associated with the archeological culture called the Nordic Bronze Age.
Around 500 BC we believe these people expanded a bit south. Archeologists call this new area the Jastorf Culture, but its generally accepted to also be Proto-Germanic.
After this, one can argue that Germanic peoples didn't cease this expansionist behavior for the next 1500 years. Initially as one linguistic culture, then later as three separate langues (East, West, and North Germanic), and lastly as Norse (Vikings and Varagians).
So I think its fair to say that this kind of violent expansionist behavior was a feature of their culture. Warring with the residents of the Swiss plateau and attempting to take their land around 100BC was completely in character for them. It was certainly not the result of any factors unique to the unfortunate Helvetians and their immediate Germanic neighbors.
This kind of behavioral continuity means we can perhaps make use of the Historical research on the cultural causes of the Viking Age to speculate about the motivations of those same peoples' ancestors. Below are some of the various theories put forth that aren't limited in scope to the Viking Age itself.
Demographics - Their culture during this period was consistently producing more population growth than their existing territory could support.
Economics - They were always looking for the next score.
Political/Technological model - Essentially, the Germanics were relatively strong militarily, and power, like nature, abhors a vaccum.
Religous - What we know about the religion of the Norse when it was finally recorded seems to imply a person had to die in battle to go to their equivalent to heaven. It seems like this would certianly encourage war.
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Many of the Indian tribes in the USA had warrior cultures, where a man could gain prestige and wealth by attacking and looting members of other tribes.
Those Indian tribes were bad neighbors to other tribes, being in a constant state of low intensity warfare with all of their neighbors except for any allied tribes.
And of course the less aggressive tribes had to defend themselves against attack and often retaliated against attackers. Thus the less aggressive tribes also honored their warriors, though they would have other ways for their men to gain prestige and wealth beside war.
So the men of warrior age of the warrior societies were not only expected to fight to defend their groups from occasional attacks, but also usually wanted to go and attack someone at least once a year to win glory and prestige and loot. What the women and children and old people in those groups felt about the constant chance of being attacked by enemies and killed or kidnapped is not known.
My guess is that as long as a warrior tribe was e winning its constant low level warfare and chasing other tribes out of their territories, most members of the tribe would think that the constant state of war was worthwhile. But if the tribe was being slowly defeated by their enemies the members would probably be less contented with the state of constant war.
In the 19th century the Crow tribe was usually loosing. Their territory was being taken over by the Blackfeet and the Sioux, and also by the eastern Shoshone. So during the the Great Sioux War of 1876-77 the Crows fought for the US government against the Sioux and even allied with their former enemies the Shoshone. After the Sioux were defeated the Crow Chief Plenty Coups said that at last the Crows could sleep at night without fear.
And the situation may have been the same with the Helvetii and the Germans. There might be almost one small raid every day, happening at some spot along the hundreds of miles of border between them. Not a constant big battle between every single warrior all along the border line almost every day. Instead almost every day there might be one small skirmish where a few warriors fought during a raid for loot.
And of course Caesar might have greatly exaggerated the frequency of such fights.
And the example of the warrior tribes in the USA shows that people could stand a constant state of low level warfare for year after year, generation after generation, and not know of or long for anything more peaceful.