It states (Tehillim 126:1) את 'ה בשוב המעלות שיר כחלמים היינו ציון שיבת, "...when Hashem will return the captivity of Tzion, we will be like dreamers."
The Midrash says that כחלמים היינו means that when Moshiach will come, we will consider ourselves to be like Yosef HaTzaddik, the (Bereishis 37:19) החלומות בעל, the person who dreamed.
When a person has a bad dream and then wakes up, he realizes it was nothing at all. It was just a dream. Just a moment earlier, he was going through the worst problems, but when he awakened, he realized it was just his imagination. This is what happened to Yosef. He went to Mitzrayim, was a slave, suffered from aishes Potiphar, and then he was sent to jail. He suffered immensely, but then Yosef became the leader of Mitzrayim, his family had a place to go to in the hunger years. Initially, it seemed bad, but when the good was visible and noticed, the suffering was viewed as a bad dream.
Dovid HaMelech is telling us את 'ה בשוב כחולמים היינו ציון שיבת, when Moshiach comes, and the galus is gathered, the entire exile will seem to us like a bad dream and not a real problem.
It states (46:4) גם אעלך ואנכי מצרימה עמך ארד אנכי עיניך על ידו ישית ויוסף עלה, "I shall descend with you to Mitzrayim, and I shall also surely bring you up, and Yosef shall place his hand on your eyes."
The Meshech Chochmah explains that with these words, Hashem was telling Yaakov that he shouldn't ask questions about Hashem's hashgachah. He shouldn't wonder why he must go down to Mitzrayim. עיניך על ידו ישית יוסף, the story that happened with Yosef will close your eyes from asking questions about Hashem's hashgachah.
The Meshech Chochmah writes, "Yaakov shouldn't wonder what purpose and benefit will come from going down to Mitzrayim... Hashem told Avraham to go to Eretz Canaan and to live there, the land that is uniquely his, but to go to Mitzrayim, what for? Hashem said, עיניך על ידו ישית יוסף, what happened to Yosef will close your eyes from thinking about this matter and from trying to understand Hashem's hashgachah, which is beyond us. Who would think that good things would come from Yosef's tzaar? Yet, Yosef became the ruler of Mitzrayim, and he taught the people of Mitzrayim his righteous ways. The entire land was under his influence. This episode is sufficient to place a hand over your eyes from thinking about Hashem's ways and trying to understand matters with your logical mind..." The story of Yosef reveals to us that everything is for the good, even when it doesn't initially appear so.
