Here is the reason why associating with the fool is bad. I would like to give more detail about it as follows:
(a) Indulgence in intoxicants which cause infatuation and heedlessness
i. loss of wealth,
ii. increase of quarrels,
iii. susceptibility to disease,
iv. earning an evil reputation,
v. shameless exposure of body,
vi. weakening of intellect.
(b) sauntering in streets at unseemly hours
i. he himself is unprotected and unguarded,
ii. his wife and children are unprotected and unguarded,
iii. his property is unprotected and unguarded,
iv. he is suspected of evil deeds,
v. he is subject to false rumours,
vi. he meets with many troubles.
(c) frequenting theatrical shows
i. where is there dancing?
ii. where is there singing?
iii. where is there music?
iv. where is there recitation?
v. where is there playing with cymbals?
vi. where is there pot-blowing?
(d) indulgence in gambling which causes heedlessness
i. the winner begets hate,
ii. the loser grieves for lost wealth,
iii. loss of wealth,
iv. his word is not relied upon in a court of law,
v. he is despised by his friends and associates,
vi. he is not sought after for matrimony; for people would say he is a gambler and is not fit to look after a wife.
(e) association with evil companions
i. any gambler,
ii. any libertine,
iii. any drunkard,
iv. any swindler,
v. any cheat,
vi. any rowdy is his friend and companion.
(f) the habit of idleness
i. that it is extremely cold,
ii. that it is extremely hot,
iii. that it is too late in the evening,
iv. that it is too early in the morning,
v. that he is extremely hungry,
vi. that he is too full.
This is what we should avoid in life. We should learn and teach these things to our young generation so that they can live peacefully and happily with themselves and others. It is not bad to himself only, but it is also bad to his family and society. Understandably, if we can avoid all these things, we can live in peace and happiness [ AN 4.62; PTS: A ii, 69]: the happiness through obtaining wealth by righteous means, the happiness through using that wealth for the benefit of himself and others, the happiness obtained by being free from debt, and the happiness obtained by being free from blameworthiness. Here, we can notice that the happiness of the layman is closely related to the concept of wealth. But wealth must be obtained by righteous ways of life; otherwise, there is no happiness among man in the world. The Buddha balances the first three happiness which comes from the good economic, with the last one, which is spiritual. He emphasized that the first three kinds of happiness is not equal to one-sixteenth part of the happiness obtained by being free from blamelessness.
By Bhikkhu Pannavaro