When did the Netherlands begin reclaiming land with their dams, and why did they decide to do so? When nobody else did to nearly their extent.

So this happened in three basic stages. The first main stage was in the 16th and 17th centuries, in which the lakes north of Amsterdam, such as the Achtermeer near Alkmaar, were drained via windmill pumping. The second stage was in the 19th century, when the Haarlemmermeer was drained (the largest such project at the time in the country), this time using steam powered pumps. And the final stage was during the 20th century, with the reclamation of the entire present-day province of Flevoland from the Zuiderzee. That's the when.

The why is more complex, but comes down mostly to the ever increasing need for arable farmland and a need for safety. Long story short, the Netherlands had a lot of peat bogs and salt marshes. The first inhabitants in these areas in the middle ages began to build drainage ditches in order to make the peat swamps suitable for farming, but this caused land subsidence (i.e. sinking). Over time, this sinking interacts with groundwater and eventually forms lakes. It was these lakes that would later be pumped out and filled in. Weather, especially harsh storms, made these areas very dangerous for pre-modern societies, what with their being so low as compared to the surrounding area (the surrounding area being lakes and the sea which were behind dykes and dams!). The easiest way to rid themselves of this danger was to get rid of the lakes, and so they began pumping the water out.

The exception to this was Flevoland. That was created out of the Zuiderzee in the mid-20th century, primarily for reasons of population. The Netherlands is a very densely populated country and in order to have enough farmland to produce the food necessary to feed folks, the government felt that more arable land was necessary - hence the so-called Zuiderzee Works, which was the entire effort of dams, ditches, and land reclamation that gave the country an entirely new province!

Source: Hoeksema, Robert. "Three Stages in the History of Land Reclamation in the Netherlands." Irrigation and Drainage 56 (2007), 113-126.