Here's why:

* Gravitational pull: The moon's gravity pulls on the Earth's oceans, creating bulges of water.

* Sun's influence: The sun also exerts a gravitational pull on the Earth's oceans.

* Alignment: During new and full moon phases, the sun, moon, and Earth are aligned. This alignment creates a combined gravitational pull from the sun and moon, resulting in spring tides – the highest and lowest tides of the month.

So, to summarize: the highest tides occur during the new and full moon phases due to the combined gravitational pull of the sun and moon.