Q:

Is using the Simple Past always considered a more polite way to ask something?

When I use the Simple Past to ask something, is the sentence always considered more polite?

I wanted to know if you would accept an invitation for dinner.

I wanted to ask you something.

I wanted to take you out for dinner.

In Italian, the first and the third sentence would make me think the person changed idea, and doesn't want to say the reason. I can imagine a person getting hungry, or offended for something done or said, announcing what was planned to do, as to say "see what you are going to miss."
Is using the Simple Past always considered a more polite way to ask something, or are there cases where the Simple Past is really used to mean something that was planned to do, and that is not going to happen anymore?

A:

Both uses of the past form are possible. Some grammarians speak of the past form as "remote", because it "removes" the verb to a distance which may be either temporal or social.

Hi, Maria, I'm calling because I wanted to take you out for dinner.

Here you use the past form to make want less demanding. The remoteness is social.

Sorry I couldn't track you down last night. I wanted to take you out to dinner.  

Here the primary sense is clearly temporal remoteness. You could, however, work in a degree of social remoteness by also employing the progressive construction, which in this case would have no implication of imperfectivity - it would just be a further distancing device. 

Sorry I couldn't track you down last night. I was wanting to take you out to dinner.  

Note that this last construction could also be used in the first example:

Hi, Maria, I'm calling because I was wanting to take you out for dinner.  

In this particular instance, however, the progressive construction would probably not be used because it clashes with the progressive calling in the main clause.  

Grammarians give this the Latin name horror aequi, "the
widespread (and presumably universal) tendency to avoid the use of
formally (near-)identical and (near-)adjacent grammatical elements or
structures" (Rohdenburg).

You'd be more likely to use it in a context without a prior progressive:  

Are you by any chance free tonight? I was wanting to take you out for dinner. 

To express the "see-what-you're-missing" sense in English you would employ a  construction with a stronger sense of intention. The past would be necessary in this case, rather than optional, because you would be speaking of a prior intention which no longer holds:

I was going to take you out for dinner, but I've changed my mind. 

