Musar do Koheleta 7:2

But just as reflection in this matter causes one to acquire the trait of Separation, so too does ignoring it cause its detriment. Frequenting the aristocrats and the affluent who run after honor and increase vanities. For when one sees their honor and grandeur, it is impossible for lust to not be roused within him to covet them. And even if he does not allow his Evil Inclination to defeat him, nevertheless he will not escape a battle and will thus be in danger. This is as Shlomo said: "It is better to go to a house of mourning than to go to a house of feasting" (Kohelet 7:2).

This is as they said regarding Rabbi Eliezer: "he would expose one handbreadth and conceal two, and was as one compelled by a demon" (Nedarim 20b). He would not derive any enjoyment whatsoever, and would perform the act only with intent to the mitzva and divine service. Shlomo said on this: "Know Him in all your ways, and He will straighten your paths" (Mishlei 3:6).

And this is why one must feel genuine pain in their soul over the passing of the righteous, and over all the afflictions of the righteous that are wounded for our transgressions and broken for our sins. And because of [impending] evil the righteous are gathered, and the pain of heaven increases since the people of faith are gone, who come [to pray] in the power of their [good] deeds, etc. etc. And one must be distressed even over the righteous who died in antiquity, as is said (Zohar Part 3 Page 57), all that mourn over the deaths of the sons of Aaron on Yom Kippur, all their sins are forgiven. And this is the meaning of “the living shall take it to heart” [Ecclesiastes 7:2] — “if a flame has fallen among the cedars, what shall the wall-hyssop do?” [Moed Katan 25b]. And further one should take to heart that due to our great sins the faithful disappear from the children of man, and the pious are lost from the earth and men have no strength. And the Holy One, blessed be He, has no people in this world to love as in the days of old and in the years of antiquity. And this is because of our sins as is said in Gemara (Sanhedrin 11a): So-and-so Rabbi was worthy that the Divine Presence rest upon him, except that his generation was not worthy of it. And because of the decline of the generations, the Torah continues to diminish, progressively decreasing. Over this, it is fitting to feel bitterness and to grieve with a writing sorrow.