Let's consider a player, whom we shall call Bob. Bob loves to be a warrior and loves exiting stories about how he's the best in killing others and about how badass he is.
Bob's pretty new to Pen & Paper, and has only played rule-less games in chats before. But: He actually is quite content with the rules, and all this dice rolling. He doesn't understand the rules, but with a bit of help all's fine. He's not whining about not being super awesome (not a lot anyway).
But: He wants to play the badass-warrior. You can see it in the way he plays and he always plays this kind of characters in the chat-games.
Problem is: He picks "non fighter" character classes. For his first character he told me the backstory, he was dropped as a baby at a fortress and grew up there, and that he already saw the "guard" class and wants that. I told him: "Oh, that's a great backstory, but guard is more like 'police'," He answered: "Yeah, I know. My Character isn't a soldiers, he was just guarding the fortress". I explained to him what that character build would be good at, and that I don't think it would fight his preferred playstyle.
He quickly grew bored of that character, when he learned that the actual fighter was a lot better at fighting, than his character. Plus he is uninterested in playing the social interactions his character would be good at. (Guard being a cross between fighter and social).
His second character, he is creating right now, is a dwarf, who hunts dragons. Me: "That's awesome, there is a character class for that: Warrior. Dwarven Warrior are specially trained to kill dragons. There is.." Bob: "Oh, no. I don't want to play a warrior. I already looked, there is big-game hunter. I want that."
Big-Game Hunter is a wilderness specialist. Again not a fighter. I already know he will be bored and become cranky, as soon as he discovers that the fighter characters are a lot better at fighting than him.
We are playing The Dark Eye, a german fantasy RPG. (Thus character classes are actually professions in this case.)
How can I help a player find the character-classes and perks, that would fit his style of play and story, if he actively doesn't want it?
I already tried creating the character with him (both times, we met to create his character) and I also told him I'd gladly create the character rules-wise. But he want's to do it himself (which is fine by me). I tried talking to him, about what the character classes are good at, and what alternatives exists, that would fit the story better.
And while it is true, that The Dark Eye will allow you to become proficient in any skill with enough XP, this takes time. The group as a whole as already complained how long everything takes, so I'm already handing out more XP.
I think he might not want to play a soldier or warrior, because he's mocked in the online games, for always playing that kind of character.