Why does it say "Ach Hevel Bnei Adam"?
Rashi: Do not fear them, since Hashem is a refuge for us at a time of affliction.
Vayikra Rabah (29:8, cited in Rashi): People say, Ploni marries Plonis - this is folly. Maharzu (on the Midrash) - people agree to marry based on their thinking - really, "b'Moznayim La'alos" - Hashem weighed how to match them, and it was already decreed before their birth!
Vayikra Rabah (29:8): The follies of children of Adam (Avraham - "ha'Adam ha'Gadol ba'Anakim" - Yehoshua 14:15), he (Avraham's merits) atones for them in the month (Tishrei) of Mazal Moznayim.
Malbim: Hevel has no substance. Also great people, who have some substance, are Kazav, i.e. something that ceases.
What is "Kazav"?
Radak, Malbim: It is like Hevel, something that does not last - "Asher Lo Yechazvu Meimav" (Yeshayah 58:11).
What is the meaning of "b'Moznayim La'alos"?
Refer to 62:10:1:2-3.
Rashi: If they would ascend a [balance] scale, they and their follies are equal.
Radak: If they would ascend a scale with Hevel [air, on the other side], they will be lighter than Hevel - "Hevel Havalim" (Koheles 1:2).
Malbim: If you see people ascending at the height of success, they are like one who ascends on a scale. This is a sign that they are Hevel (for the light side rises and the heavy side descends); they themselves have no substance.
