The Once-ler has, in what he later in life described as "a completely irrational sense of optimism". He firmly believes that he is going to change the world, that his product is revolutionary, and that the future is going to fantastic! Any attempts by others to dissuade him from this, are met with genuine shock and disbelief. He feels bad for these dream-squashers, but, he'll show them all. The Thneed will change the world!
By and large, he's a genuinely nice guy, if a little self-centered.
He eats up praise. Since he didn't hear enough of it as a kid, even the slightest compliment gets him flustered and delighted. He especially likes to be told how great his ideas and creations are.
The Once-ler is a dreamer. He likes to invent things that look cool (although his definition of "cool" doesn't always mesh with the rest of the world's.) and he puts fun before practicality. As a result a lot of the things he makes are more showy than functional.
He can't see disaster until it's too late. Planning ahead is not the Once-ler's strong suit. He's not a particularly brave man, but, he doesn't look before he leaps.
Power and the Once-ler are a very, very, bad, totally "ungood" mix. Power goes right to his head. He does not handle it well, and his worst personality traits (namely greed and pride) take over. He is not the man you want to give unlimited magical powers to---disasters would rain down. (And he would later, feel genuinely awful about it, and beat himself up for decades.)
Luckily, the Once-ler is a complete non-believer in magic. Even having met the Lorax, a living, breathing, ancient magical being, he has to see something in action before he'll believe it.