system
Given the following post, respond as if you're commenting on Reddit:
human
[BttF] Why does Marty have such a problem with being called a chicken?

In the third movie he gets called "yellow" instead but either way, he has a major issue with people calling his courage into question. Why?

As much trouble as he gets into everyone that happens, you'd think he'd let it go - and at the end of the third movie, he does just that by not racing. Why not sooner?
gpt
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.