Why do I need to add capacitance to crystal oscillator?

This depends on the crystal oscillator specifications and the factors in the circuit to determine that the same 16MHZ crystal resonator, the load capacitance value may be different, such as 10pf,20pf .... The load capacitance value is determined during the production process and cannot be changed. When purchasing crystal oscillator, you should be able to obtain accurate specifications.
When the crystal oscillator is used in the circuit, the CL=C+CS should be satisfied.
CL is the load capacitance value of the crystal oscillator in the specification book,
c is the external capacitance value of the circuit (generally by two capacitors through a series-parallel relationship),
CS is the distribution capacitance of the circuit, which is related to the circuit design, component distribution and other factors, the value is uncertain, generally 3 to 5PF.
Therefore, according to the above formula, we can approximate the capacitance value that should be used, and this capacitance value can make the crystal work near its nominal frequency.

such as: I use 430 SCM, 8M Crystal oscillator, with 12pF capacitance, in fact, the size of the capacity is not necessary more accurate, a few skin method to more than 10 skin method can be. Probably all the same, you look at the chip data should have.

Why should the external crystal oscillator in the monolithic microcomputer have two capacitors?? Why do you want to receive it?

1. In order to satisfy the conditions of the resonance.    specifically: The load capacitance of the crystal element refers to the total external effective capacitance across the crystal at both ends of the circuit. is the capacitance required for the normal oscillation of the crystal oscillator. The general external capacitance is to make the equivalent capacitance at both ends of the crystal oscillator equal to or close to the load capacitance. Not all crystal oscillator circuits require a matching capacitor. Whether it needs to be determined by the form of an oscillating circuit, the equivalent model of the crystal should be used for analysis.  
2. grounding: two capacitor grounding next to crystals ,  ,  .  ,  The input and output of the oscillator pin are reversed-phase ,  But from both ends of the parallel resonant circuit, quartz crystal , 

Of course, you can also understand this:

the nominal value of the crystal oscillator is tested with a " load capacitance " condition that satisfies this condition at work and the oscillation frequency is consistent with the nominal value, i.e., Only by connecting the appropriate capacitance to meet the vibration of crystal oscillator requirements, crystal oscillator to work properly.

Why do I need to add capacitance to crystal oscillator?