What is the main reason we have four seasons on Earth?

Seasons are caused because the Earth's axis is tilted, so at some parts of its orbit the northern hemisphere points towards the Sun, and receives concentrated sunlight, whilst the southern hemisphere is pointed away and receives spread out sunlight. Six months later the situation is reversed. However, the division of the year into four seasons is somewhat arbitrary, and applies mostly to temperate and polar zones. Tropical regions often have different patterns, such as a wet and a dry season.