I can't relate. Why do you think that a single level of a class doesn't support a character's identity?
I don't want to speak for him, but I feel the same way and I can explain why I feel that way.
A class, to me, encapsulates someone's entire life and personality. When I think Wizard, I think of someone who spent their entire youth pouring over tomes, reading magical theory for years, likely apprenticed to some old wizened man who forced the youth to do house chores before he'd teach him any more this week. I think of a nerdy, bookish type who is too weak to wear armor and therefore walks around in robes. He commands the power to change the universe but is wise enough to know when and how to use that power.
That entire image is smashed when Wizard is a class that can be dipped into for one level in the middle of adventuring. It instead creates the image of the big strong fighter who spent his life learning to use a sword, training as part of the city watch, defending the city against Orc hordes, spending his nights with his fellow guards in the tavern drinking ale to celebrate their victories, who studies a spellbook in his spare time for months and months and says "Well, I learned 2 spells but I never really wanted to be a Wizard so I'm done with that now, back to guarding the city using my sword."
It just seems like the equivalent of someone in real life saying "I know I've been a firefighter all my life. But I've decided to go to law school during my spare time. Once I become a lawyer, however, I'm just going to keep my job as a firefighter and use my law knowledge only whenever I get a parking ticket."
It seems like anyone who studies something does so because they either want to switch their profession permanently away from their old one or are interested enough in their new area of study that it continues equally along side their original profession.