The Y'rushalmi says (Bava Kamma 7:7):
א"ר יוסי בי ר' חנינה כל המגדל כלב רע בתוך שלו עליו הכתוב אומר (איוב ו יד) למס מרעהו חסד
Rabbi Yose the son of Rabbi Chanina1 said: "The following verse applies to anyone who raises a bad dog in his residence (Iyov 6:14): 'For a tax of kindness from one's fellow....'"
When a person mentions his good fortune, it is customary to add that he was able to appropriate some of the divine kindness designated for his fellow and take it for himself by raising a bad dog - an act of extraordinary self-sacrifice and kindness that makes other people look bad by comparison.2
1 Rabbi Yose had personal experience in appropriating the good fortune of his brother, Rabbi Chama bar Chanina. They were both sons of Rabbi Chanina bar Chama, and both were destined to experience particularly miraculous good fortune due to their relationship with their illustrious father (per B'rachos 57a, "הרואה... חנינא חנניא יוחנן נסי נסים נעשו לו"). However, Rabbi Yose became extraordinarily great by raising a bad dog and usurping his brother's share of their good fortune (Rav Assi testifies to Rabbi Yose's greatness in Bava Kamma 42b, "אמר רב אסי האי מילתא מפי דגברא רבה שמיע לי ומנו רבי יוסי ברבי חנינא").
Rabbi Chama, however, could not but nostalgically recall his father's good fortune and great wealth that allowed him to fund the construction of the synagogues of Lod (Y'rushalmi, Pe'ah 8:8 and Sh'kalim 5:4, "רבי חמא בר חנינה ורבי הושעיא רבה הוו מטיילין באילין כנישתא דלוד אמר ר' חמא בר חנינה לר' הושעיא כמה ממון שיקעו אבותי כאן"). Thus, Rabbi Chama lamented the severity of the "bite of a dog" that robbed him of his destined good fortune (Y'rushalmi, Rosh HaShana 3:9, "ר' חמא ב"ח... ולא דמיא נשיכת כלב יראה נשיכה נחש והביט", which the Korban HaEida ad loc. explains to mean that it was more difficult for a person to be healed from a dog bite than from a snakebite by looking at the copper snake).
2 Cf. the explanation of R' Avraham the Convert in Tosafos (Kiddushin 70b, s.v. קשים): "Rabbi Avraham the Convert explained that since converts are knowledgeable about the commandments and are careful with them, they are difficult to the Jews like leprosy (ספחת), for due to their conscientiousness, the Holy One Blessed be He recalls the sins of the Jews by contrast when they do not fulfill His will." Note that the gematria of למס מרעהו חסד (plus the gematria of the letter preceding and the two letters following that phrase) equals the gematria of ספחת (plus the kollel).