Testimony of a Professional Gambler
The Mishna tells us that a dice-player (or other gambler) is disqualified as a witness in Beit Din. Rebbe Yehuda qualifies this, stating that a dice-player is disqualified only if he has no other occupation. The gemara explains that such a person "doesn't occupy himself with settling the world" (Sanhedrin 24b). This isthe ruling of the Shulchan Arukh (CM 34:16).
What exactly is wrong with someone who "doesn't occupy himself with settling the world"? Where are we charged with such an obligation?
The answer is that lack of an occupation is not a disqualification in every individual, but specifically for a gambler. The mentality of gambling is in effect the opposite of a productive, constructive approach to livelihood. The ideal approach is to seek a livelihood in which a person's income is derived from some benefit he provides to others; the gambler by contrast, earns all his income at the expense of others. Ideally a person recognizes that a livelihood is earned through effort and "hishtadlut"; the gambler, on the contrary, experiences that all his ups and downs are dependent on the "luck of the draw".
Now we understand the disapproval of the gambler, but why is he disqualified from testimony? The Tur draws a concise and profound connection between the specific character flaw of the professional gambler and an inability to testify:
"Because he doesn't occupy himself with the settlement of the world, to know how much a person has to exert himself for money, and it is a light thing in his eyes to testify falsely to cause a loss to his fellow man" (Tur CM 34).
A witness in court must be conscious of the immense gravity of his testimony. On the basis of his words, the judges will make their judgment and determine who is entitled to the sum in dispute. A person accustomed to an "easy-come, easy-go" approach to money just doesn't see why it matters who wins. From his point of view, the court judgment itself is nothing more than the "luck of the draw".
There is a deeper connection between these issues. While we have to make an exertion, a hishtadlut, ultimately our livelihood comes from HaShem, "for poverty and riches are not from a profession, rather all is according to merit" (Mishna end of Kiddushin). Judgment too is ultimately the provenance of G-d, "For justice belongs to G-d" (Devarim 1:17). However, human beings are bidden to take an active role, a partnership, in these processes. A person who denies or neglects his role as a partner in the process of creating and distributing wealth is in danger of similarly denying or neglecting the importance of his role as an active participant in the system of justice. Thus a person whose only economic activity is gambling is in danger of taking lightly his responsibility as a witness in Beit Din.