So I'm like, Why not say "I said?"

I listen in fascination to people in their teens and twenties talking like this:

So she's like, "Why don't we talk a long walk?"
And I'm like, "No!"

The noteworthy thing here is that "I'm like" seems to mean almost the same as "I said", and "I'm like" feels more complicated to say - to me anyway - so I figure that "I'm like" does not mean exactly the same as "I said."

But what's the difference? Here's my suspicion: "She said" is a good prefix for relating somebody's words. "She's like" is the prefix for explaining that somebody said something and all their body language echoed their words.

Comments, please? I'm, like, really curious about this.