Because, along with the rest of the human race, they were born separated from God, unable to experience God as he originally intended us to. This is because we're not good people, and since God is perfect, he can only accept perfect people.
But that's the bad news. The good news is that God went to great lengths to let us know him and the true joy he gives. He sent his son, Jesus, to live the life we were supposed to live. And as a human, Jesus died the death that was supposed to be our punishment.
Jesus took credit for our very imperfect lives so we could take credit for his perfect one. And if we ask God (as individuals), he will accredit this perfectly moral life to us, and we will never again be separated from him.
This is why they can remain strong. Their greatest need has already been met.