Why do particles not decohere in their native state? As I've been trying to wrap my head around the principles of decoherence and quantum behavior I am left wondering why fundamental particles are 'allowed' to exhibit quantum properties even in ideal conditions ( close to absolute zero and in a 'box'). 
If a particle/photon behaves in a probabilistic superposition state as seen in the double slit experiment then wouldn't we expect the multiple coincidental states of the particle to interact with each other and thus cause decoherence? This would in turn lead to an innate instability of any system above a zero point energy thereby making quantum properties not identifiable at all..but indeed we do have verifiable and replicable evidence of quantum mechanics. 
 A: 
If a particle/photon behaves in a probabilistic superposition state as seen in the double slit experiment then wouldn't we expect the multiple coincidental states of the particle to interact with each other and thus cause decoherence?

There is a misunderstanding in this statement. In the single photon at a time double slit experiment, a single photon appears as a dot on the "screen", it is not spread out in the interference pattern.


Single-photon camera recording of photons from a double slit illuminated by very weak laser light. Left to right: single frame, superposition of 200, 1’000, and 500’000 frames.

It is the accumulation of photons that shows an interference pattern, the probability density distribution for the quantum mechanical problem "photon impinging on two slits".
The probability density distribution is the square (Psi*Psi) of the quantum mechanical wavefunction.
In addition, superposition of wavefunctions can give interference effects, but there is no interaction, as in two laser beams interfering. There is no measurable photon-photon interaction, but there is superposition of the wavefunction of the two photons, and the sum gives the interference terms in the probability density distribution (Psi1+Psi2)*(Psi1+Psi2).
