It's a choice. Any Guardian can learn to use any subclass. In fact, any Guardian can learn to use powers of any class. A Titan could theoretically create a Golden Gun, a Hunter could shoulder charge, and a Warlock could create a Ward of Dawn. There's nothing barring them from learning these things. The difference between classes and also subclasses is largely a cultural one and is dependent on personality. A Guardian who is curious, daring, and crafty would most likely find it easier to imbue a knife/staff with arc light or a gun with solar light. They would enjoy patrols and adventures beyond the wall much more than studying or training. They would likely befriend others like them. Therefore, that specific Guardian would be molded into the Hunter archetype. Titan-like Guardians would likely join the Titan Order of their liking. Would-be-Warlocks would join the Warlock Order that interests them most. Because of that, they'd learn the subclass path most akin to that order. The Praxic Fire would teach upcoming Warlocks the Sunsinger and Dawnblade paths. The Firebreak order would likely teach the Striker path. Hunters would learn on their own or from word of mouth/other Hunters. Not to mention Pahanin's Errata. Before the City became organized and before Guardians became institutionalized, Guardians/Risen didn't follow strict paths. It was only after social groups and cliques of Guardians arose did classes become a thing. Subclasses can be switched on the spot, but each Guardian has their preference.

As far as Warlocks go, it depends on personality. Voidlocks are all about keeping calm, mental focus, and studying. A lot of Lore suggests they're introverts and fine with isolation and have a crap ton of secrets. Nova bomb is (lore wise) an extremely hard move to pull off correctly but it is arguably the most powerful super. Voidlocks are all about being mentally balanced and in tune with oneself and the Light. Also they're supposed to be somewhat morally ambiguous hence why Toland is kind of a dick Dawnblades/Sunsingers are about intensity and pressing forward. Sunsingers were/are about using the Light to rez themselves so they can always keep going. Contrary to Voidlocks, Sunsingers are very welcoming to company which is why they have the power to buff (to a degree) their teammates. Dawnblades model themselves after angels and use the Sword of Justice™ to wipe the battlefield clean. And Grace to also heal others **Edit: It should be mentioned that Dawnblades are typically taught how to be a Dawnblade. 1 on 1 seems to be the norm. And you have to somehow "earn" your sword for your super Stormcallers are meant to be a mix of Voidwalker and Sunsinger. All the intensity of a Voidwalker but with the buff benefits and clearing power of Solar. All about focusing in the middle of battle while being active then unleasing chaos upon your enemies.

I believe personal choice. Some may possibly be more drawn to an element than others, but I do believe that 90% of the chosen subclass is... well I just said it. By choice. Would be interesting to look into however. Wondering why Guardians choose this element or that, but that's going very deep into the rabbit hole. We might find ourselves in some type of messed up theory and/or endless paradox, haha.

Personal choice? Besides that, some very skilled Guardians can seamlessly transition between subclasses, such as Commander Zavala.

Bump?

There is hunter lore about the difficulty in mastering void subclasses. You have to be absolutely fearless.

I want to see this idea discussed. Honestly. I've yet to see proof one way or the other that guardians are stuck to one element other than the player guardian.