Why did the late Roman Army need to forcibly conscript recruits rather than having citizens volunteer for service? There is even mention of people mutilating themselves so as escape conscription. Was it because it paid less to be a soldier? Or was it because the Roman people had gone soft? Was Christianity a factor? Or was it because soldiering was considered a death sentence?
Can you go into a little bit more what you're seeing that's giving you the impression that the army of the Principate was a volunteer force? Recruitment had moved away from Italy after Tiberius ended Italian conscription (Brunt estimates that under Trajan only 1% of the legions were drawn from Italy), but conscription continued throughout the imperial period. It appears to have been the main way in which the army recruited troops, and more recent scholarship like Ian Haynes' Blood of the Provinces is increasingly showing that the distinction between citizen and provincial in the army was actually not very pronounced, with many citizens being conscripted or joining the auxiliaries and many provincials being conscripted to fight in the legions.