The scene at the end of the first film, replayed at the beginning of the second, raises a number of questions. In this scene, Doc assures Marty that "you and Jennifer turn out fine," and that the problem is that "something has got to be done about your kids." After he takes Marty and Jennifer to visit the future, however, these statements don't match up with what we see. The middle-aged Marty from 2015 hasn't exactly turned out fine, and only one of Marty's kids needs help. Why did Doc mischaracterize the situation?
It should be noted that the filmmakers have stated in interviews that they created this final scene before they had any real plans for sequels. As such, Doc's statements were probably just throwaway lines they found amusing at the time. But that doesn't explain why, when working on the second and third film, they chose to contradict Doc's statements.
The entire scene was re-shot for the second film, ostensibly because the actress who played Jennifer in the first film--Claudia Wells--had been replaced by Elisabeth Shue. Somebody on a message board pointed out to me a subtle difference between the two versions of the scene, which are otherwise quite identical. In the Claudia Wells version, Doc's line "you and Jennifer turn out fine" is spoken smoothly and confidently. In the Elisabeth Shue version, Doc visibly hesitates before saying the line, and he utters it a bit too rapidly to sound convincing. (You can examine the difference with this video. Pay particular attention to 1:25-1:38.)
Still, Doc's behavior in either version doesn't make much sense. Why is he interrupting Marty's life only hours after Marty returned from the first adventure just so they can rectify something decades in the future? Why doesn't he let days, or weeks, or months pass before asking Marty to come with him? Since they have a time machine, what's the rush? If my theory is correct, Doc is trying to keep Marty away from the present. That makes sense given what we learn in the third film: the pivotal car accident is supposed to occur later the same day.
You might ask why Doc doesn't just tell Marty straight out about the accident. He could say, "You know, Marty, later today you'll be involved in a car accident that permanently injures your hand and destroys your musical aspirations, all because you took a guy's dare." The reason for Doc's silence is explained during a brief exchange in the third film, when Doc blurts out the truth:
Doc: Marty, you can't go losing your judgment every time someone calls you a name! That's exactly what causes you to get into that accident in the future.Doc realizes that if he interferes directly with Marty's future by warning him about the accident, Marty might manage to avoid that particular catastrophe, but he'd retain his habit of acting foolishly whenever somebody calls him "chicken." Then, with his future thrown into flux, something even worse might one day occur as a result of that habit. The only safe way for Doc to improve Marty's future is by inspiring him to grow out of the habit. And what better way to do that than to show him the consequences of his son's acquiescence to peer pressure?
Marty: [Suddenly stops and turns toward Doc.] What? What about my future?
Doc: [Realizes what he just said.] I can't tell you. It might make things worse.
Marty: Wait a minute, Doc...what is wrong with my future?!
Doc: Marty...we all have to make decisions that affect the course of our lives. You've gotta do what you've gotta do. And I've gotta do what I've gotta do.
In other words, taking Marty to 2015 to help Marty's son was basically a ruse. Ordinarily, Doc wouldn't concern himself with a matter so remote from their current lives. He hopes that Marty, by seeing what happens to his son, will begin to reflect on his own weaknesses.
Of course, Doc thinks they'll go straight home after dealing with the son. He doesn't anticipate the chain of events that lead them to 1885. But by the time Marty gets back home at the end of the third film, he has in fact matured enough to avoid the accident on his own, as Doc had hoped, though influenced by different experiences than what Doc originally had in mind--a different route to the same destination.