When you actually look at Ed’s character development throughout the series, the stuff in Rush Valley with Winry was a huge important turning point for him. Moreover, his respect for the simple heroism of human beings as opposed to the power of alchemy is really very inspired by how he views Winry as a hero who does work more valuable than what he does as an alchemist. He looks up to her and realizes she has skills that alchemy can’t compare to and that’s why she’s a big factor in his final decision.
First off, we have to look at the aftermath of the Nina thing. This was the first time Ed had to accept that he can’t solve everything with alchemy.
Here he blames being an “insignificant human” as the reason he couldn’t save Nina. He still believes he should be able to solve everything with alchemy and sees his humanity as his weakness here. He feels like a failure because he believes as an alchemist, his skills should apply to any situation.
He has a similar crisis when he’s unable to use alchemy to get to the doctor when the woman in Rush Valley is giving birth. He tries to build a bridge, but it collapses under it’s own weight.
Ed has a similar line at a different point in the Brotherhood episode that adapts it, and I remember that it someone at Mark Spoils commented it came off as kinda manpain-y, like he’s suddenly making this situation about his issues. I’m always one to criticize that sort of thing and I understand with the condensed-ness of the Brotherhood episode it can come off that way, but reading the full chapter the line was there for a very specific reason- despite having all the fantastic superpowers at his disposal, Ed is helpless in this situation.He has no power to save anyone.
Which makes it a powerful statement when Winry isn’t. Winry takes the reigns and saves the day not with superpowers, but with courage and basic medical knowledge. Ed may know alchemy and fighting, but when the situation is an everyday crisis of normal human proportions like giving birth, it’s Winry who can get things done and save lives. Winry doesn’t rely on alchemy, so she has important skills that Ed doesn’t, so she can save the day where alchemy can’t.
And that’s when Ed changes his attitude. Through Winry, he sees that “insignificant humans” are amazing and capable of things alchemy cannot possibly do. He would not have had the revelation if not for Winry’s heroism.