What is it with metaphors, really?

Definitions

(Because I'm not even sure what these terms mean half the time.)

simile (vertaus)
as stable as Windows

metaphor
  All the world's a stage,
  And all the men and women merely players;
  They have their exits and their entrances;
(William Shakespeare, As You Like It)

allegory
Allegory of the cave
Stanley Kubrick's 2001: A Space Odyssey (???)
Edward Scissorhands, Asperger's Syndrome
Though it is similar to other rhetorical comparisons, an allegory is sustained longer and more fully in its details than a metaphor, and appeals to imagination, while an analogy appeals to reason or logic.

analogy
contrast homology

...Reading the stuff above confused me as much as it cleared confusion.

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All metaphors break down at some point; they stop applying to the issue the metaphor was made for, which is my main reason for not liking them. I never know when to stop analyzing the metaphor so as to keep its message clear. I nearly always stop only after I've managed to break it.
I also have a high chance of not getting metaphors or even similes. Maybe the fault is not always in me, but I have no way to be sure.

I don't get most allegories. Allegory of the cave is one that I do get (or so I think) and I think it's fairly brilliant. I have no idea about 2001: A Space Odyssey – allegory, where? I think I understand Edward Scissorhands, but I'm not sure – and if I do understand it, it breaks down in an annoying manner. Edward is weird enough without the scissors.

Usually metaphors and similes help people understand things that would otherwise need much more explanation to be understood. I would rather have the explanation, but both are probably even better. These are usually useful even to me, especially in physics.

Even more annoying than over-analyzing is when I simply fail to understand the metaphor altogether. Often enough I can ask what it means or I'm familiar enough with the other subject that I'll just believe that they are somehow similar. But when even explanations fail, I just feel really annoyed at not getting it. Usually the metaphor is there to supposedly simplify the issue – and I'd have more likely understood the unnecessarily complicated explanation about it.