Why does a delta often form where a river meets the ocean?

A delta doesn't always form near an ocean, but yes, it often does. It forms from deposition of sediment. Sediment is the process of wearing or erosion of a natural object. The water slows down as it nears the ocean.

When river water is no longer flowing downhill, the water slows down. This makes sediment drop to the bottom. Sediment deposited where a river flows into an ocean or lake builds up and a landform called a delta is formed.