Choosing to follow Harry wherever the other boy led was not a decision that Neville took long to make. Actually, he hadn't even had to be asked by Harry himself—when Luna told him that Harry would be better off if he agreed to 'go on a (possibly deadly) adventure', Neville agreed on the spot.
It wasn't that he thought Harry was without fault. Over the course of their Hogwarts education, over the course of the war that followed, Neville became very, very aware of the imperfections inherent in everyone. But all the same Harry was one of the few people Neville knew that really, truly, completely wanted to do the right thing. He might fail sometimes, sure, and he might stumble when things got rough, but Harry rarely chose to do something just because it might make his own life better—his every action seemed specifically decided upon in an attempt to help others.
Neville wasn't like that. Neville wanted to be liked, wanted to be respected, wanted to be left alone, wanted to be included—he had a hundred internal motivations that wrestled and fought for control over every decision he had to make. All of that wasn't necessarily bad, but it did mean that his imperfections were, to him, perhaps a little less forgivable than Harry's own.
Case in point: Harry was apparently (accidently, of course, because the other Gryffindor really did have the worst luck) the 'Master of Death' —a title which was, shocker, a misnomer. No, Harry had no more control over life and death than he'd ever had before becoming its so-called master. Instead Death, or at least a being identifying itself as such, had control over Harry, and could use him as a puppet to do as the deity wished.
And the deity wished for Harry to play hero all over again.
All of this had been explained by Harry as he helped Neville trim a recently acquired devil's snare, worded as if Harry couldn't quite believe the words coming out of his mouth himself.
Neville had no problem believing it. Why would it not be true, after all, in a world which had prophecies and time travel and hocruxes and magic?
He'd accepted it as the truth the second he was told it, told it by a boy who'd never quite managed to lie convincingly a day in his life.
And so, merely three years later, Neville found himself sitting against a lean-to beside his older sister Kisha as his mother gave birth to her third child with the help of her sister, a midwife, screaming in wretched pain while her husband—his father—paced, white-faced, outside.
"Antwan! Antwan!" Kisha snapped.
"What?" Neville asked.
"Have ya been list'ning to a word I've bin sayin?" The four year-old asked.
"Well, list'n now." She waited until Neville sat up and faced her. "You're gonna be a big brother now, so there's a whole buncha rules you gotta know. You gotta protect them, okay? And make sure they're doing good. We're family, ya know, so we gotta look out for each other."
Neville blinked. "I can do that." It would be years before he could be any help to the others—Merlin knows they weren't born anywhere near him—so here was something else he could do, something he could do because it was the right thing to do rather than any real or imagined reward. "I'll be the bestest big brother ever."