Periods of abstention from food appear in traditions with no shared origin. The convergence is explained by properties of the practice itself rather than by any common doctrine.
Everyone has access to it
Fasting requires no money, no object, no building and no literacy, which distinguishes it from almost every other formal observance.
A practice that the poorest member of a community can perform identically to the wealthiest is unusual, and traditions have often noted the point explicitly.
That accessibility is part of why fasting survives in communities that have lost their institutions, buildings and texts.
It marks time unambiguously
Abstention divides a period into before, during and after in a way that is impossible to overlook or forget partway through.
Traditions use this to mark seasons, preparation before a major festival, and mourning, where an interval needs to be set apart from ordinary time.
The marking works because the body registers it continuously, which no symbolic observance achieves in the same way.
It synchronises a community
Communal fasts are observed by everyone simultaneously, which produces a shared condition across a whole population for a defined period.
The meal that ends a daily fast becomes a fixed gathering, and traditions with such fasts typically build substantial social practice around that moment.
Shared abstention followed by shared eating is a strong pattern for cohesion, and it recurs in traditions that share nothing else.
The rules define what the practice means
Traditions differ in what is abstained from: all food, particular foods, food during daylight, or food alongside other activities.
Exemptions are similarly detailed, and almost every tradition excuses children, the elderly, the ill, travellers and those who are pregnant or nursing.
The care taken over exemptions indicates that the practice was understood as demanding, and that it was never intended to apply without regard to circumstances.
Restraint is the point rather than deprivation
Where traditions explain the practice, they usually describe it as training in restraint rather than as denial valued for its own sake.
Texts across several traditions criticise ostentatious fasting directly, which suggests the display problem arose early and often.
The same sources tend to pair fasting with giving, tying the food not eaten to the food provided to someone else, which converts a private practice into an obligation to others.