There’s barely a discouraging word to be heard in Dave LaMattina and Chad Walker’s documentary “I Am Big Bird: The Caroll Spinney Story,” the title of which is pretty self-explanatory. Oh, sure, the puppeteer, now an octogenarian, had a turbulent relationship with his father, who understandably thought that his son’s fascination with puppetry was impractical, and his first marriage ended in divorce because his wife found his dedication to his craft incompatible with marriage. The death of Muppets founder Jim Henson, moreover, was a terrible blow—the song that Spinney, dressed as Big Bird, sang at the funeral (shown in full here) is a poignant reminder of that.

Spinney ultimately reconciled with his dad, however, and his second marriage has had an almost fairytale quality. And while Henson’s death was undoubtedly painful, the company he founded has flourished, and Spinney remains active in it after forty-five years. And virtually all of the rest of “I Am Big Bird”—a collage of archival material, including stills and footage (home movies, show clips, behind-the-scenes moments) interspersed with excerpts from interviews with friends and fellow workers as well as his loving wife Diana—is almost invariably cheerful, despite the fact that his other great Muppets role was as Oscar the Grouch. (Given Spinney’s endlessly sunny personality, the notion suggested by some interviewees that Bird and Oscar represent his two sides, light and dark, comes across more as poetic license than reality.)

In short, what the documentary presents is a picture of a life that appears almost blessed. (Big Bird was supposed to fly on the ill-fated Challenger space shuttle to generate interest in the flight among kids, but the costume was too large and Spinney was replaced by Christa McAuliffe.) Spinney was hit with desire to be a magician and puppeteer early on, and his loving mother encouraged it. His schoolmates teased him about it, but he persevered, and after a stint in the military joined the cast of a local “Bozo’s Big Top” show (one of his co-stars remembers him fondly). Then lightning struck when Henson saw him at a puppet festival and offered him a job in spite of the fact that his act suffered from technical problems.

The rest, as they say, is history, and “I Am Big Bird” covers it all, including the gestation of “Sesame Street” and Bird’s enormous popularity, which even led to the production of a feature, “Follow That Bird,” in 1985. There’s also coverage of the Henson crew’s trip to make the special “Big Bird in China” (1983), which leads to one of the more obvious bits of manipulation on the part of LaMattina and Walker, who arrange a reunion between Spinney and the Chinese woman who, as a young girl, was his co-star in it. Spinney himself projects a childlike “gee whiz” quality that apparently hasn’t failed him even into his eighth decade, though even he seems to have been nonplussed by Mitt Romney’s announcement that as president he’d end funding for PBS although he said he loved Big Bird—a remark that became an issue during the 2012 campaign.

The documentary might have sacrificed some of the commentary by others about Spinney in favor of more footage of his actual performances and greater emphasis on what a difficult role Big Bird actually is. (It requires Spinney to control the head with his extended right arm for long periods while hitting his marks without being able to see them on the stage floor.) One might also have liked to hear him talk more about his feelings concerning the escalating popularity of the Elmo character, who rivaled Big Bird among the tykes watching “Sesame Street.”

But the big problem with the documentary is that while nice—arguably too nice, in fact—it feels like fifty minutes of material drawn out to nearly ninety. The repetition becomes tiring, even when the subject is such a pleasant man.

Of course, Boomers who grew up with Bird and are nostalgic to return to their childhoods may well disagree with that assessment.