When is a sentence like "mi sono mangiato una pizza ai quattro formaggi" used?

Question

I was trying to explain to a friend of mine (who is not Italian) the differences between these sentences, but I discovered I may not.

Mi sono mangiato una pizza ai quattro formaggi.

Ho mangiato una pizza ai quattro formaggi.

I was thinking the first was used as emphasis, but I have the habit to use the first instead of the second sentence, which means I am overusing that kind of sentence, or it's not really used as emphasis.

Is the first sentence really used as emphasis, or does it have a different use?

I know that a beginner in Italian should probably stay away from the first type of sentences, but my friend asked why I use the first sentence when she would expect me to use the second one.
Probably, if I were able to find the equivalent of mi sono mangiato una pizza ai quattro formaggi in American, I would help my friend better, but I would like better to give an explanation of when to use the first type of sentence.

Answer 1

The new edition of the Devoto-Oli dictionary, edited by Luca Serianni and Maurizio Trifone, devotes a "Questioni di stile" box to this phenomenon:

Il verbo mangiare viene spesso usato con un pronome atono con valore intensivo: mi mangio un gelato. Mentre la frase mangio un gelato descrive l’azione in modo neutro e oggettivo, la frase con il pronome atono esprime una maggiore partecipazione del soggetto all’azione, quasi a sottolineare il piacere con cui si gusta il gelato. Questo uso espressivo dei pronomi atoni mi, ti, si, ci, vi con i verbi transitivi, inizialmente molto esteso nell’italiano regionale del Centro e del Sud, è oggi diffuso in tutta Italia ed è ormai pienamente accettato. Nei tempi composti il passaggio dalla forma attiva a quella pronominale comporta il cambio di ausiliare: ho mangiato un gelato / mi sono mangiato un gelato.

(Summarising: mi mangio un gelato has an intensive value, expressing a greater participation to the action, as if to highlight the pleasure being had in eating the ice cream. It is a construction initially found in Central-Southern Italian, but now it is spread in the whole of Italy.)

More information can be found in the article about “Dativo etico” of Treccani's Enciclopedia dell'italiano, which talks of a “valore affettivo-intensivo, atto a segnalare una più attiva e sentita partecipazione del soggetto all’azione”. See also the article about the “Verbi pronominali”, that is, on the verbs that for various reasons (reflexive etc.) are constructed with a personal pronoun.

Answer 2

I understand what you're trying to say, but in reality, you may be making it more difficult for yourself than it has to be.

IT to EN:
Look at the literal translations (here's where we get a laugh at languages and have a little fun):

"I ate (myself) a four-cheese pizza..." vs. "I ate a four-cheese pizza..."

Depending on where you are in America, you may hear both expressions, including the first one with the "myself" added to it (Southern, middle american dialects).

Simplified, it is just an informal vs. formal kind of unwritten rule. I sometimes find myself using "bad Italian" when I'm speaking with family/close friends, but at the same time, I'm not saying it's "bad italian," because in language we have a tendency to utilize different expressions within context, comfort, and habit. Just like there is no such thing as "bad food" from a nutritionist's standpoint, there is no such thing as "bad Italian" -- well, in this case. I hope this helps Ciao, Raffaele