This is the Bohr-model of Carbon.
When you look at the periodic table, you can see that Carbon has 6 protons, 6 electrons and 6 neutrons.
The 6 electrons are divided among the shells around the nucleus.
In the first shell, there are 2 electrons.
The second shell can hold eight electrons, but there are only 4 electrons.
Carbon can bind to 4 other atoms, and this is the reason!