You'll notice that this wasn't an issue for Marty when he first traveled back to 1955. He appeared normal and was presented as a young man who rarely took the opinions of others into account.
But then he started fiddling with the timeline.
Time, in this reality, acts upon the continuum in broad waves; a person who briefly removes him- or herself from it will observe time "correcting" itself (photographs changing, newspapers rewriting, even people disappearing).
But what they won't see is their personality being altered. Each time Marty traveled through time, any action he took in the past had an inevitable reaction on his disposition in the future - this becomes increasingly pronounced as his story progresses, culminating in his ability to turn away from Needles.
"But we went into the future and saw that Marty already had that accident, and this was before he tried changing anything!"
Quite right, but as Doc Brown said, you aren't thinking fourth dimensionally, and certainly aren't considering the behavior of time.
See, Doc Brown himself returns to 1985 and informs Marty that it's his kids that get into trouble, but Marty himself is fine. Having traveled immediately that far into the future, time hadn't yet 'caught up' to 2015 and corrected itself after the shenanigans in 1955, thus Doc Brown observes a Marty McFly who has no recollections of ever traveling through time.
But by the time Marty himself jumps into 2015, the corrections have asserted themselves: Marty has been changed, perhaps subtly, into a man hellbent on the adoration and acceptance of his peers: a sellout, a loser, a chicken. It is no surprise that Marty gets into his accident with the Rolls Royce the very same day that he returns from 1955. In this timeline, he has no time to even begin to understand that he's been altered.
But, in the reality we observe, Marty gets to drag out this single day in 1985 by many additional leaps through time, both forward and backwards. These adventures continue to have sweeping effects on his personality, but - luckily - he eventually overcomes his new flaw and sidesteps the incident with Needles, allowing him to start a new path into the future, likely one more-closely mirroring that which Doc Brown initially observed.
But then he started fiddling with the timeline.
Time, in this reality, acts upon the continuum in broad waves; a person who briefly removes him- or herself from it will observe time "correcting" itself (photographs changing, newspapers rewriting, even people disappearing).
But what they won't see is their personality being altered. Each time Marty traveled through time, any action he took in the past had an inevitable reaction on his disposition in the future - this becomes increasingly pronounced as his story progresses, culminating in his ability to turn away from Needles.
"But we went into the future and saw that Marty already had that accident, and this was before he tried changing anything!"
Quite right, but as Doc Brown said, you aren't thinking fourth dimensionally, and certainly aren't considering the behavior of time.
See, Doc Brown himself returns to 1985 and informs Marty that it's his kids that get into trouble, but Marty himself is fine. Having traveled immediately that far into the future, time hadn't yet 'caught up' to 2015 and corrected itself after the shenanigans in 1955, thus Doc Brown observes a Marty McFly who has no recollections of ever traveling through time.
But by the time Marty himself jumps into 2015, the corrections have asserted themselves: Marty has been changed, perhaps subtly, into a man hellbent on the adoration and acceptance of his peers: a sellout, a loser, a chicken. It is no surprise that Marty gets into his accident with the Rolls Royce the very same day that he returns from 1955. In this timeline, he has no time to even begin to understand that he's been altered.
But, in the reality we observe, Marty gets to drag out this single day in 1985 by many additional leaps through time, both forward and backwards. These adventures continue to have sweeping effects on his personality, but - luckily - he eventually overcomes his new flaw and sidesteps the incident with Needles, allowing him to start a new path into the future, likely one more-closely mirroring that which Doc Brown initially observed.
