The saint does not look for rest but for tribulation. He knows "tribulation works out endurance, and endurance, tried virtue, and tried virtue, hope. And hope does not disappoint." This is parallel to what Jeremiah says: "I have called on tribulation and misery, for your bitter word was to me joy and gladness." In this world I desire nothing but tribulation that I may have happiness and repose in the next. That is why, he says, "I now bear with bitterness, that afterwards I may have all sweetness." The people of the Lord coming out of Egypt came to Mara, which means "bitter," and from Mara into Sinai, which means "temptation." Again, Jeremiah says, "I sat alone because I was filled with bitterness."
Source: HOMILIES ON THE PSALMS 39 (PS 114)