It is not just a person's physical constitution, their intellegence, their education, or even their social conditioning that enables them to withstand hardship. Much more significant is their inner development. And while some may be able to survive through sheer willpower, the ones who suffer the least are those who have a high degree of patience and courage in the face of adversity.
There are a number of qualities which are important for mental peace, but from the little experience I have, I believe that one of the most important factors is human compassion and affection: a sense of caring.
Most REAL change is slow. Happiness and joy are the purpose of life. If we know that the future will be very dark or painful, then we lose our determination to live. Therefore, life is something based on hope.