Why do Tides occur regularly?

Tides are caused by the gravitational pull of the moon and sun. The moon has a bigger influence than the sun because even though the sun is vastly bigger than the moon, the moon is much closer. So, high tides occur when the moon is directly overhead. The highest tides happen when the sun and the moon are in line because then the gravitational force of the two bodies are acting together and mariners call these tides, "Spring tides" (Nothing to do with the season) When the moon is at half phase, the tides are lower and mariners call these tides "Neap tides." There are two high tides in each 24 hour period because of the earths 24 hour rotation period. It's a little hard to explain without a diagram, but if you want to know more about it, I expect that you will find a full explanation including diagrams, on Wikipedia or some other website.