Rashi: This is simply a man.

Rashi citing Tanchuma: It refers to Hashem - "v'Al Demus Kisei Demus k'Mar'eh Adam" (Yechezkel 1:27).

Ibn Ezra: This is one who toiled in worldly matters with his great Chachmah, and attained his heart's desire that he pursued.

Rashbam: Did any man toil as much as me in Chachmah, Da'as and Kishron (monetary acquisitions)?!

Rashbam: It is [toiling] in monetary acquisitions.

Rid: It is with Kashrus and Emunah. Metzudas David - then it is proper that he keep his acquisitions until he dies, and bequeath them to his children. However, it is not so!

Ibn Ezra: It is Po'el Yotzei with two objects (he will give it to him). It is like "Ki Eretz ha'Negev Nesatani" (Yehoshua 15:19).

Rashbam: Hashem will give [his share] to [one who did not toil].

Ibn Ezra: It is the portion he receives via Shamayim's 'lottery'; children, lifespan and food (income; they depend on Mazal, and not on merit - Mo'ed Katan 28a).

Seforno: [I ceased to hope that I will merit to leave my acquisitions to a Chacham,] for experience shows oppositely - one toils to acquire acquisitions, and his portion and Mazal he bequeaths to one who did not toil at all, and sometimes the latter inherits Bal Korcho (against the will) of the Morish.

Metzudas David: He gives his acquisitions to one who did not toil for them, and is not proper to inherit him.

Rashi: This is Dor ha'Mabul - "Rabah Ra'as ha'Adam ba'Aretz" (Bereishis 6:5).

Rashbam: The first one toiled for Hevel (for nothing).

Seforno: Havah Lei is the lowest matter - one is pained over a world that is totally not his. Ra'ah Rabah is that Mazal sometimes goes against the Morish's desire.

Metzudas David: He toiled without benefit, and must give it to another in his own lifetime.