Was it??? Although the tribes were moderately stable political entities, Gaul as a whole tended to be politically-divided, there being virtually no unity among the various tribes. Only during particularly trying times, such as the invasion of Caesar, could the Gauls unite under a single leader like Vercingetorix. Even then, however, the faction lines were clear. (wiki)
Caesar didn't just build the bridge to cross the river he built it to show and to warn the Germans just what they were up against, to show the professionalism, speed and superior technical ability that the Romans possessed. It was not about crossing the River it was about impressing and frightening the Germans.
OK, in theory the Germans were at peace with Rome, but they were make bigger and more daring raids into Gaul and onto the tribes that Caesar had promised to protect.
OK, avenging ?? maybe not entirely. He went to Alexandria in pursuit of Pompey, to end the Civil War once and for all. But I don't doubt for a second that when he was presented with the head of Pompey he was not sickened by the sight, after all Pompey was a great Roman and ex son in law, and to be murdered in this way by "barbarians" was totally disrespectful and would surely of made any Roman swear revenge. What happened after was not entirely related to what happened to Pompey but it DID happen and Caesar DID come away with another victory. not for just him but for Rome as well.
Caesar was a master of propaganda and used it to further his career and reputation, there weren't many Romans who didn't use these "standards"