Rav Sadya Gaon: Few people remember the days of their lives, unless Hashem arouses him and helps him to benefit and rejoice.

Rashi: There is not length of days in this world; the days are few. Why should one toil to gather wealth? He should toil for something that will stand for him in the world to come!

Ibn Ezra #1: If his good does not last a long time, he should remember the days when he was content; there is Simchas Lev in this memory.

Ibn Ezra #2: He should remember that the days of his life are not many ¹ . "Es" is extra (Yemei Chayav is the subject, and not the object).

Rashbam: One who remembers, will not remember him long after his death. Even the one who inherited his property will not remember him.

Ri Kara: He should remember that his days will be few, therefore he should benefit from his property (Metzudas David - and do Tzedakah and Chesed with it, amidst joy) while he is alive.

Rid: This is tied to verse 18. He should remember that his days will be few, therefore he should rejoice in his lifetime.

Seforno: This does not usually happen, that together with acquiring wealth, he will have days of Chayim (good. Refer to 5:19:2:7.)

Rashi: Hashem is fixed testimony for this world. Ma'aneh is a noun, for there is a Segol under the Nun, like "Ein Ma'aneh b'Fi Sheloshes ha'Anashim" (Iyov 32:5). Koheles Zuta applies our verse to Elkanah, who guided Yisrael to ascend to Shilo for the festivals. The path that he led them this year, he did not lead them another year, in order to publicize the matter ¹ and accustom them to come. Therefore, it says "v'Alah ha'Ish ha'Hu me'Iro mi'Yamim Yamimah" (Shmuel I, 1:3). Rashi - the Midrash expounds that Hashem testified about him in Tanach.

Ibn Ezra #1: He supplies. This is like "E'eneh Es ha'Shamayim v'Hem Ya'anu Es ha'Aretz" (Hoshe'a 2:23).

Ibn Ezra #2: Ma'aneh is like Anah, acceding to the request, like "Ya'aneh Es ha'Kol" (10:19). Its grammatical form is like "Ma'aleh Nesi'im" (Tehilim 135:7). Hashem helps him to be happy - "v'Hishlito Le'echol Mimenu" (18).

Rashbam: Hashem [did not give to him wealth to bequeath it, rather,] to make him rejoice in his portion. Ma'aneh is Hif'il conjugation, to make rejoice, like "va'Ta'an Lahem Miryam" (Shemos 15:21), "v'Ansah ha'Shirah" (Devarim 31:21).

Ri Kara: When Hashem gives to him money, it is as if He testifies - I gave to him this money, in order that he will rejoice in it and benefit from it.

Rid: If Hashem will accede [that he rejoice]. "Ki" is like Im, like "Ki Yikarei Kan Tzipor Lefanecha" (Devarim 22:6).

Seforno: Normally, at a time of Simchas Libo (when he succeeds to get wealth), Hashem afflicts him with worry about efforts [to keep his wealth], and he does not benefit from it.

Metzudas David: Do not say, who knows what is hidden in the heart, whether or not I did with Simchah. Hashem knows thoughts; He is a witness if you acted with Simchah.

Rashi: In order that ¹ he will rejoice to do good in his lifetime.

Rashi, based on Midrash Zuta: [Elkanah] rejoices during the Regel.