User blog comment:The ASL Pirates/Haoshoku Haki Theory/@comment-24850639-20150217071851

I find it interesting that so far all the confirmed users are pirates, no marines or revolutionaries. A big part of me doesn't want to believe its linked to family heritage since we still seem to know so little, but Garp's comments about Luffy having "inherited" it when he used it at Marineford kinda shuts that down. Perhaps I'm not looking in the right places, but it just seems hard to find the proof of that, at least if we're talking directly from parent to offspring. Dragon hasn't been confirmed to have it, despite wide belief it isn't proven that Roger had it, and it isn't proven that Donquixote Homing had it. But at the end of the day Haki is will, and "inherited will" is a rather big theme of the series, so if not direct parents then perhaps someone else that'll be explained later.

The only real commonality I've been able to see so far are the stressful situations that cause it to manifest itself. Ace did it to save his little brother, Mingo was being tortured, Motoboro incident aside Luffy was trying to save innocent girls from being killed, and I think that covers all the first time uses we've witnessed so far.