(According to the owner of Paupoli’s)
As everyone knows, the Romans conquered the Greeks and, like always, enslaved them. The Romans then proceeded to make the enslaved Greeks their teachers, which was their big mistake. The Greeks, in order to avenge themselves upon the Romans, taught and wrote only in their native tongue. Consequently, the Latin language slowly dissolved into Greek.
Now for the second half of the story: Why there are two forms of Greek—ancient and modern. After converting the Romans to speaking their language, the Greeks discovered that they no longer could use their Greek to communicate without the Romans being able to understand it. So they created a second language, closely related to Greek, but different enough that Greek-speaking Romans couldn’t understand it. And that was the origin of modern Greek.