The Once-ler liked to remind himself that his family loved him. Even if they forgot, all the time. (All the time. All the time.)
He is three, five, seven, when his father disappears, instead of before he was born. His brothers and his aunt and his uncle like to hold this over his head. They like to pretend his father left because of him. (He thinks that this isn't so, it isn't so, and he almost believes it. Almost.) His mother never talks about his father. It is a forbidden topic, the Once-ler's father.
(Once-ler never asks why. Why did he leave, why didn't he stay, why weren't we good enough for him? (Though the last one is secretly why wasn't I good enough for him?))
Yes, he did like to pretend his family cared, though they never liked him much. Or at all. There is a lot of pretending in his formative years. There is a lot of time to think in those years, too, for all the lack of attention his family paid to him. And for all that time to think, out came inventions - failed ones. Of course, his family says, of course they failed. It's all they are: failures. Just like you, isn't it funny? You're just like your inventions. He almost begins to believe it. (This is a lie. He does begin to believe it, at least for the majority of his childhood. There is no "almost" about it.)
He is nine when he realizes his family has abandoned him. He is nine and it is the middle of the night and he is locked out of his own home. He is nine, it is cold, it is raining, and he is trying to rationalize all these things his family has done to him. (They love him, don't they?)
It works for a very long while.
He has a tree house, when he's eleven. His brothers have grown tired of it, as they always do. (Most of the Once-ler's things are second-hand. The only thing that isn't is that old bass guitar his father left behind.) He doesn't care that it's second hand, this time, because his family wouldn't bother him up here. (Not that they ever cared what he did.)
The tree house lasts him five years, until he can't make it any taller. His mother calls him a weed, and he isn't quite sure what she means by it. (Then again, he is never quite sure what his mother means by anything. She could have meant he was growing like a weed, or that he was a weed. He'd rather it be the first.)
The Once-ler is sixteen, in a tree house that is now far too small for his height, when he thinks of the thneed.
The thneed lasts him two years instead of five, or at least the idea of it. (He has a jingle for it in one. It's a work in progress.) He thinks that it might be made of truffula leaves.
And the tree house--
The tree house is a safe space, at this point, more for hiding from his family than it is for anything else. (If one were to look inside years later (that is, if it were theoretically still standing), they would find two years' worth of planning and planning and planning. All for what, they might wonder.
All for what, indeed.)
So the Once-ler hides in his tree house, planning and planning and planning for two years, accumulating other inventions if not to keep his family from getting too curious. He starts ignoring his family's taunts the year between his seventeenth and eighteenth birthdays. He stops believing them, for that one year. (And almost another one, if not for--well. That comes much later in our Once-ler's life.)
He is seventeen and a quarter when he first mentions the thneed. His family laughs and taunts, as they always have - though, this time, he is less fazed by it and pretends they're trying to be supportive; upset him so he'll want to prove them wrong out of spite. Of course. (This rationalizing business has worked wonders. It's worked eight years, though something is telling him it might be unhealthy.)
His father's bass guitar needed to be tuned, and it needed new strings, and it needed to be cleaned. (The jingle is still a work in progress, but he's getting there. He's getting there. The guitar will get him there.) It is the only thing of his absent father he has. He liked to think that his father would have brought him along if he'd already been born when his father left. There are a lot of things the Once-ler liked to think about his father.
His family liked to pretend he was why his father left. He didn't want to believe it, though something in him did. Something in him knew that his family wasn't pretending to an extent, either. (They never pretended to hate him and taunt him and mock him, but the Once-ler doesn't realize this until much, much later in his life. He is still young and naïve here, mind you.)
After his eighteenth birthday, halfway to his nineteenth, the Once-ler sets off with nothing but his father's bass guitar, Melvin the mule, a cart, and of course, the basic needs for survival. His family waves him off (good riddance, he thinks he hears his mother saying some distance away, and Aunt Grizelda's cruel laughter following), and he tries pretending again that they care.