simple, complex, compound sentences with "so" and "because".

Oh I am having truble with classification of sentences. As I remember from primary school our teacher tought us that, if not sure what type of sentence we are dealing with, we should ask a question: does one clause expands on the information given in the other clause? Does it describe it in any way? If it does, than we know that we are dealing with complex sentence. I try to apply this method now and it turns out I misunderstand "so". What is the difference in meaning between:
Jenninfer was heavily hangovered and behaved like a bear with sore head, so I decided to clear off as quickly as I could.
and
Because Jenninfer was heavily hangovered and behaved like a bear with sore head I decided to clear off as quickly as I could.
Am I wrong or "so" does suggest that my short stay at Jennifer's place had something to do with her being in bad mood? If Jennifer had been nicer, I would have stayed longer. Therefore sentences with "so" should be considered complex no compound.
I guess I have similiar problem with "for". Both "so" and "for" suggest the relationship between two actions and causation of some kind, so why the sentences made up with them are considered compound and not complex ones?

[Mister Micawber]

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It would be a lot easier to simply remember the coordinating conjunctions:

for, and, nor, but, or, yet, so

It is these that join compound sentences; the other conjunctions join dependent clauses on to independent clauses to produce complex sentences.
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