Explanation: Roman legionaries often were enthusiastic about going on campaign - campaigns meant plunder and opportunity to prove one's valor and advance in the ranks! Only... some campaign destinations are more desirable than others.
Hispania, for some 300 years, was a bloody ulcer in Rome's side. Rome exercised control over it, but only loosely, with Hispanian guerillas subjecting Roman troops to constant attrition, little glory, and, perhaps worst of all, no clear enemy to loot and enslave. How terrible! Yet all the same, Roman armies were (begrudgingly) sent into Hispania again and again and again to subdue it, each time emerging with little progress to show for it.