How do I write dialog where list items are prefixed with letters?

Punctuation is a matter of style, and as such you should be guided by your manual of style. I use the Chicago Manual of Style, which by explanatory text and examples makes the following recommendations for in-line lists:

• Do not use a colon to introduce a list that is a complement or object of something in the preceding text. I think we can extend this to the subordinate clauses subject to because.
• Use parentheses to separate the label from the list items. CMS encloses the label -- (A) she makes me laugh. But there seems no good reason not to prefer a single, following parenthesis. I suppose some complication should arise if the list times themselves contained parenthesized elements, but your example avoids this.
• Separate simple list items with commas. Semicolons separate related independent clauses not connected by a conjunction, so avoid semicolons unless the list items have internal commas:

  I love her because A) she makes me laugh, although, to be sure I have no sense of humor, as she herself well knows; B) she's very caring, so caring that I have often been moved to tears, sighs, and moans; C) ....

So the question remains what to do about such lists that appear in direct discourse (i.e., as quotations of speech). You wouldn't change the punctuation if the introductory verb was to write:

I wrote, "I love her because A) she makes me laugh, B)...."

so why would you change anything when using to say? Spelling out the letters will only lead to confusion: when you get to Dee, your reader is sure to wonder who that is. By analogy, if you used numbers as labels:

"I love her because 1) she makes me laugh, 2)...."

You wouldn't be moved to write

"I love her because One) she makes me laugh, Two)...."

And when you tell people about Sue Grafton's first alphabetical novel A is for Alibi, you're not confused about pronouncing the title, even in a quote:

I asked, "Did you like A is for Alibi?"

So there's every likelihood that anyone reading your examples would be similarly unconfused.

I don't know there is one rule that applies to this. However, I would write the quoted list using the actual letters, not the phonetic representations, and I would use commas to suggest the pauses that would typically occur in the speech, e.g., "I love her because A, she makes me laugh, B, she's very caring, C, she really gets me as a man, and D, I love her family." I recommend using commas as punctuation because, if I were using words instead of letters to demarcate items of the list, commas would be appropriate, e.g., "I love her because, for one thing, she makes me laugh, secondly, she's very caring, thirdly, she really gets me as a man, and, most importantly, I love her family."

As an aside, if I was your editor, I would suggest you substitute the last letter of your lettered list with a word or words, e.g., A, ..., B, ...., C, ... and "lastly," or "the biggest reason," or "what makes her a blessing most of all is, " I think listing items in speech comes across more believable and authentic when it's not fitting a rigid pattern.