Rashi: Every exertion in work will have a profit.

R. Yonah: This verse arouses Zerizus. One will always find gain via toil, be it little or much, whether in Chachmah or work.

Malbim: The primary reward for visiting mourners is for silence (Brachos 6b). For every evil that comes upon man, he must be sad in his heart, and remember that it came due to his sins; he must repent in his heart. This is mourning over the dead! "Tov Lalecbes El Beis ha'Evel

Yerushalmi Brachos 4:1: Since [Chanah] prayed excessively, she caused Shmuel's years to be shortened! She said "v'Yashav Sham Ad Olam", and Shmuel] lived only 52 ¹ years.

Rashi: Words of folly cause only a loss.

R. Yonah #1: One will always find loss through idle talk; it usually causes idleness. Idle talk is attributed to the lips - "Amarti Ach Devar Sefasayim" (Yeshayah 36:5). It is not proper to be attributed to the heart.

R. Yonah #2: Devar Sefasayim is glorifying himself over the deed that he intends to do. The deed depends on Hashem. It is not in man's hand! His intent trespasses in relying on his Binah and strength.

Orchos Tzadikim (Sha'ar Leshon ha'Ra DH Im Gilah): One who sins may not reveal another's sin. Surely he does not reveal another's hidden matters for good, "Ach l'Machsor."

Malbim: There is gain and reward for sadness, in the heart. However, if one verbalizes complaints that Hashem did so to him, there will be a loss. "Va'Yidom Aharon" (Vayikra 10:3) - he received reward for his silence.