Not only resilience to the challenges, but resilience to the fact that some people you will just fail to help. The more you care about those in your care, the more it will generally consume you when they don't succeed.
This is a hugely difficult thing to overcome, and it's a big reason why, for example, hospital doctors typically end up with relatively little empathy for patients: it's very hard to maintain your mental health while empathizing with people hurting and dying every day. You really have to build some kind of wall between you and them to cope with the inevitable losses.