Doc: Now remember, according to my theory you interfered with your parents' first meeting. They don't meet, they don't fall in love, they won't get married and they won't have kids. That's why your older brother's disappeared from that photograph. Your sister will follow and, unless you repair the damages, you'll be next.
Geez, Doc. Lay it on us. Don't sugarcoat it or anything. All right, so Doc is direct and honest, and we can appreciate that. Basically, he's informing us of the first major transformation in the film, which is brought about by Marty bumping into his parents and changing the course of history. This transformation isn't an emotional one, as much as it is a your-hand-is-going-to-start-disappearing-and-then-the-rest-of-you sort of deal.
George: I'm George. George McFly, and I'm your density. I mean…I'm your destiny.
Smooth. George may come across as a tad awkward in the flirtation department, but this is actually a big step up for him. He never would have done anything like this before Marty came along, so he's exhibiting some definite growth. And, lucky for him, Lorraine actually likes "awkward and helpless." As evidenced by her Florence Nightingale issues.
George: No, Biff, you leave her alone.
Whoa. Where did this George come from? The old George would have followed orders from Biff, closing the door and leaving Lorraine to her fate. But the new George has courage, and integrity… and he's growing to like this Lorraine gal. We can already see the formation of a better, more assertive human being.
George: Well, Marty, I want to thank you for all your good advice, I'll never forget it.
Here George acknowledges the role Marty played in turning him around. It wasn't just an inner voice that told him to go after Lorraine, or to tell Biff to "get his hands off her." It was this mysterious, wonderful stranger who came unexpectedly into his life and set him straight. Hm. Maybe he should name a kid after him…
Marty: Doc, Doc. Oh, no. You're alive. Bulletproof vest… how did you know? I never got a chance to tell you. What about all that talk about screwing up future events, the space-time continuum? Doc: Well, I figured, what the hell?
For better or for worse, Doc's transformation is a lightening up on the subject of interfering with the space-time continuum. He used to be hard-nosed on the issue, but a while after he tore up that letter from Marty, he had second thoughts and pieced it back together. Good thing, too, or else they'd be piecing him back together.
Lorraine: Oh, I sure like her, Marty, she is such a sweet girl.
This is quite a 180 from "I don't like her, Marty." However, since her dating experience became completely altered after meeting her son a dozen years before he was born her views on teenage boys and girls have changed. Now she's totally okay with "parking" and "girls chasing boys." We like this new Lorraine. Very progressive.
Doc: Roads? Where we're going we don't need roads.
The DeLorean wants to undergo a transformation of its own. Everybody else is doing it…