I'm just wondering, what causes the existence of quantum levels?

[alxm]

Seems you're just disagreeing on what 'fundamental' means.

The HUP is 'fundamental' in the sense that it's a basic, and very general result of QM.
But it's not 'fundamental' in the sense of being a fundamental postulate QM is built on.
You have to have QM to derive the result.

Sure, it makes for a nice heuristic which can be used to explain why atoms are stable, but you're just explaining one QM result in terms of another then.
Similarly, you could explain the atom in terms of the 1d particle-in-a-box, showing that the energy levels increase as the size decreases,
and thus it's this 'confinement energy' which balances the nuclear attraction. (and as an added bonus, you also explain the Rydberg formula)

 

[docpangloss]

The argument seems to have changed into a discussion of uncertainty, but as to the OP, the electron is held in the atom by potential energy differences. If you accept that electrons have De Broglie wavelengths, they can only exist around the electron at specific radii, otherwise they would destructively interfere with themselves and stop existing, which causes the discrete levels observed. This is, of course, somewhat of a simplification, but the concept is there.

 

[LostConjugate]

The argument seems to have changed into a discussion of uncertainty, but as to the OP, the electron is held in the atom by potential energy differences. If you accept that electrons have De Broglie wavelengths, they can only exist around the electron at specific radii, otherwise they would destructively interfere with themselves and stop existing, which causes the discrete levels observed. This is, of course, somewhat of a simplification, but the concept is there.

Then what is the mediator between the electron's wave function and the electron itself that tells the electron to stay put?

 

[docpangloss]

The electron is indistinguishable from its wave function. Electrons are not strictly particles or waves, but exhibit characteristics of both. The wave function is just a convenient representation of the square root of the probability of the electron being observed in a given location.

 

[LostConjugate]

The electron is indistinguishable from its wave function. Electrons are not strictly particles or waves, but exhibit characteristics of both. The wave function is just a convenient representation of the square root of the probability of the electron being observed in a given location.

So we know why the electron is held in place and does not crash into the nucleus though we don't know how. There is no observable holding it in place.