Having done a fair bit of psychology I think that war in the ancient world would have been “easier” than modern combat for a number of reasons:

1. society was a lot more violent – we do not have public executions any more whereas they would have grown up in a society where seeing someone tortured/executed may not have been such a huge deal. This would make seeing a similar thing on the battlefield as less of a shock.
2. the societies tended to glamorise war a lot more and so Post Traumatic Stress Disorder (PTSD) may have looked different (this is a psychological condition that occurs after people experience a traumatic event – it involves a number of symptoms including flash backs etchttp://www.behavenet.com/capsules/disorders/ptsd.htm). I read somewhere that even in the American Civil War these flashbacks of experiences were described as “war nostalgia” (which kind of has connotations of remembering good experiences). I find it interesting that the first time (AFAIK) that combat fatigue/PTSD became a major issue was around WW1.
3. In antiquity then the actual battles were much shorter and so they weren’t exposed to long periods of stress (e.g. battles that go on for weeks) except in the case of sieges.
4. I suspect that nowadays some people have a greater regard for human rights/suffering etc and so it is psychologically more problematic to inflict suffering on another human (and therefore has the potential to be traumatising).
5. It may have been less likely that you would survive from a wound back then (so if you were injured then you would die rather than going on to develop PTSD).
6. back then I think that they would have had a bit more us vs them mentality (viewing the enemy as subhuman barbarians dehumanises them and so stops you feeling sorry for them).
7. Soldiers were probably not required to think beyond the fight (i.e. nowadays there are more “rules” to warfare which means that soldiers are required to think about the enemy as humans).