Brachos 43b: Hashem made everyone happy with his profession. This is like people say -- hang moss for a pig [for him to eat it], and it will do its thing (roll it in the mud).

Rashi: At a time of good, it is nice that good reward come for a good deed. At a time of evil, it is proper that payment come for a bad deed.

Ibn Ezra: Man dies in his old age, and everything occurs in its time, according to the supreme Chachmah.

Rashbam: Hashem arranged all times nicely.

Ri Kara: Everything that man toils in, when he does it, it seems good to him - Simchah at a time of Simchah, and mourning at a time of mourning.

Rid: All these times that He made - "Es Laledes

Rashi: The Kesiv is Chaser, without a Vav, like Helam (hidden). If man knew that he will die soon, he would not build a house or plant a vineyard.

Ibn Ezra #1: Olam also refers to time or eternity - "Elokei Olam Hashem" (Yeshayah 40:28). This is like Elokei Netzach or Elokei Kedem. The same applies to "umi'Tachas Zero'os Olam" (Devarim 33:27).

Ibn Ezra #2: It is like early Chachamim said (the world). It refers to worldly desire.

Rashbam: It is time.

R. Avigdor #1: The Kesiv is Chaser, like Helam - the day of judgment is hidden ¹ .

R. Avigdor: The Kesiv is like Helam. The Gematriya of "Gam Es ha'Olam Nasan b'Libam" equals that of 'Ki Helim Shmo ha'Mefurash.'

Rav Sadya Gaon: He put in man's nature the continuation of the world and dealing with it. Man does not realize what Hashem did from the beginning to the end.

Rashi: He put Chachmah of the world in people's hearts - a little in this one and a little in this one, so man does not know all Hashem's deeds, and does not know when he will die and in what he will stumble. This is so he will put to his heart to repent. He will worry - today or tomorrow I will die. It is good that Hashem hid the day of death from people (refer to 3:11:2:1)!

Ibn Ezra: People engage in the world as if they will live forever.

Rashbam: He knows that there are times of good and of evil.

Ri Kara: People are happy at a time of comfort. One builds a house and plants a vineyard, and rejoices in seeing a world of Simchah.

Rid: He causes them to engage in the world and its Hevel.

R. Avigdor: Hashem put in the heart of craftsmen to build building and plant tree; each does according to his skill and profession ¹ . It is nice in his eyes, whether it is [objectively] clean or ugly (refer to 3:11:1:1). The world needs perfumers and tanners ² (Kidushin 82b).

Seforno: He put in his heart axioms, via which man can grasp other matters. This proves that [man's intellectual Nefesh] is Nivdal (non-physical), and can judge Kelalim beyond the power of physicality. However, his intellect does not understand the details like Nivdalim (purely spiritual beings).

Metzudas David: The ways of the world and its benefits are in people's hearts to understand them fully, if they will delve into them greatly.

Refer to 3:11:3:1-2.

Ibn Ezra: It is due to his engaging in the world.

Rashbam: Not all times are good, and not all are evil. If they were, man would not repent. Why should I abandon my evil deeds? What benefit will I get from my Teshuvah?

Ri Kara (2, 11): Hashem hid from man what will be in the end. If he knew at the beginning what will be at the end, he would not rejoice at a time of Simchah, nor mourn at a time of mourning ¹ .

Rid: They do not understand that there is no gain from matters of this world. If they did, they would refrain from engaging in them!

Seforno: Man's intellect is inferior, and does not encompass all His deed, therefore he makes many efforts without benefit.

Rashbam: From the beginning of time to the end.

Metzudas David, Metzudas Tziyon: He can understand part of Hashem's deeds, but not fully. "Mi'Bli" is like Milvad [other than].