Disclaimer: I've mostly studied the architectural & urban history of Venice, but as there is only 1 other response to this post, I'm going to gove it a go. Mods, feel free to remove.

The venetians were quite proud of the stability and longevity of their republic. "La Serenissima" referred to a number of factors that kept the peace in the city from internal disputes as well as invasion. This can be narrowed down to a well-fed citizenry, a stable liberal oligarchy and a standing navy combined with its strategic location.

1_ The city kept a close eye on the grain reserves in case the trade routes to Venice were ever disrupted, or their suppliers were ever hit with famine. As you probably well know, a hungry mob is an angry mob, and angry mobs generally tend to break things and de-stablize.

2 _ An explict and conservative code of conduct checked the behavior of the upper class . Laws were applied equally to noble and commoner, and punishment in many cases was harsher for the nobleman. While there was some disparity in wealth among the nobili, many of them were merchants or bankers, aligning their interests with the citizens they ruled. The highest offices required the nobleman to spend large amounts of personal wealth, meaning that while only the wealthiest could rise to the peak, the highest office wasn't always desirable. The most successful of the Venetian upper class never became as obscenely wealthy as the best Florentine banker, partially due to this policy. Because the nobili imposed so many restrictions on themselves (including raising taxes on patricians and cutting government salaries during hard times) and would-be tyrants were prosecuted, common citizens of Venice were content to let the ruling class rule. Combined with #1, it meant that Venice, among the Italian city states, was remarkably free of riots.

3_ For external foes, Venice was always a difficult to target. Its grain reserves meant that a land seige could lead to a longer, riskier venture. Being a group of islands, it could only be attacked directly by boat, but the tides and waterways were notoriously difficult to navigate, especially for non-natives.Venice was also one of the few Italian city states with a standing military force (common practice at the time was to hire foreign mercenaries or Italian condottieri). The navy knew the lagoons well, and it was centered on a state-sponsored innovation center that implemented best maritime technology in the most streamlined production process of its time - the Arsenale. The building of both naval and merchantile vessels was considered a state endeavor, and the Arsenale adeveloped firearms at an early date, giving the Venetian navy many technological advantages. Venice was the premier naval force in the Mediterranean for many years, and found its biggest challenger in the Ottoman navy.

So the Most Serene Republic of Venice was relatively untroubled by internal and external conflicts. Venice did see some existential threats (Ottoman conflict, plague) and a couple doges put to death, but compare that to the cycle of factions, riots, sacks, occupations, and general uncertainty in Milan, Rome, Florence and Naples in the same period.

Sources:

Venice and its Merchant Empire, Katheyn Hinds
Venice: Hinge of Europe, 1081-1797, William H McNeill

Edit: formatting