Actual reason behind four seasons
It is the tilt of the Earth that causes the Sun to be higher in the sky during the summer months which increases the solar flux.
Does this tilt of Earth matter to weather? Let us understand this by taking a piece of paper and a flashlight. Shine the light from the flashlight straight onto the paper; an illuminated circle will be seen on the paper. All the light from the flashlight is illuminated in that circle. Now, slowly tilt the paper. We observe that the circle elongates into an ellipse. All the light is still in that ellipse, but the ellipse is spread out over more paper. The density of light drops as the amount of light per square centimeter drops (the number of square centimeters increases, while the total amount of light stays the same).
This is true in the case of Earth. When the sun is overhead, the light is falling straight on you, and so more light (and therefore more heat) hits each square centimeter of the ground. When the sun is lower in the sky, the light gets more spread out over the surface of the earth, and less heat (per square centimeter) can be absorbed. Since the Earth's axis is tilted, the sun is higher when you are on the part of the earth where the axis points more towards the sun, and lower on the part of the Earth where the axis points away from the sun.
The actual reason for the seasons is the tilt of the Earth's axis, for without the tilt in the axis of the Earth (at 23.5 degrees), we would have no seasons and there would be no change in our climate. The amount of sunlight energy would remain constant, then each location on earth would have 12 hours of sunlight and there would be same temperature through out and same weather every month. Some areas would always be cold, and stay cold always. Some always humid, some mostly rainy for ever!
Every area in the world would not experience four seasons. As the tropics are located in the center of the globe, they do not experience the effect of the Earth’s tilt, also they cannot experience the extreme seasonal changes that occur at Northern and Southern Hemispheres. The tropical rain belt (ITCZ - Intertropical Convergence Zone) is mainly responsible for different seasons in the tropics. It switches between the northern and southern tropics, resulting in a dry and a wet season.
In general, the summer will be warmer than winter (in each hemisphere) due to the hot Sun's rays that hit the Earth at a more direct angle during summer than the rays that hit during winter. Also during summer the days are much longer than the nights. During the winter, the Sun's rays hit the Earth at an extreme angle, and hence the days are very short. Such effects arise only due to the tilt of the Earth's axis.