system
Given the following post, respond as if you're commenting on Reddit:
human
[Pacific Rim] Raleigh said in the beginning being Drift compatible is a "Unique skill", just how rare is it?
gpt
In untrained individuals, it appears to be very rare. A mindmeld not only causes the pilots to re-experience the entirety of each other's memories almost instantaneously during the drop, while drifting it also either requires both people to have either a similar mindset, or at the very least a rather deep mutual understanding co-pilot's mindset, as they aren't really going to be functioning independently from the moment they drift. There's also the moral factors that after a drift, your co-pilot essentially knows all of your secrets, and it appears that a brain changes after a meld to become closer to a co-pilot's. (Side note:This can be seen after Raleigh's first meld with Mako when he's fighting with Chuck in the hallway; he's relying less on his own physical strength and instead fighting with a far more technical aspect in hand to hand combat. And this wasn't something that he was consciously thinking about, he was enraged, but still did it as if it were instinct).

That's why close siblings are innately better at drifting; they were raised in the same household with the same values, probably went to the same school, have experienced many things together and with regards to any secrets, they won't pick up on many new ones, and most likely not any big new ones. Essentially, there's a lot less data that their brains have to handle during a drop, so far fewer things that they could get confused by. 

Raleigh and Yancy Becket were both veterans from the earlier days of the Kaiju War, where humans were on their heels and as a result it was paramount to get Jaegers and pilots out in the field as fast as possible. Considering that, the fact that they fought the same way, had experienced the same things, knew the same things, and thought the same way put them very high on all candidate lists. Pilots being ready to deploy fast were likely so rare that the Jaegers themselves were custom tailored to their pilots, rather than any potential enemies they might face. 

Later, when the Jaegers started winning all over the place, literally dozens of battles in a row, getting pilots out fighting right now became a lower priority, as candidate applications likely skyrocketed and Jaeger production became more streamlined. By then, a more important factor had become to train the candidates optimally, and that's when they could finally start considering their pilot pairings. In this aspect, Chuck Hansen was probably at least partially right; the Jaeger Programme probably failed due to sub-par pilots, as they were simply bringing any pair that could simply just function while melding, not having the luxury of getting the best individual candidates.

A trained drifter, on the other hand knows what memories, emotions and secrets to just immediately disregard and let go, either through training or instinct. Hercules Hansen is experienced enough that he can channel his experience and love to successfully drift with his son after having lost his brother, while Marshal Stacker Pentecost appears to be either so experienced or cold that he can ignore anything his co-pilot might throw at him, while sending very little the other way. That's why by the end of the war, it appears to be so much more common in candidates that have likely been hand-picked from a pool of tens of thousands (if not millions) of candidates, as they were looking for candidates who were good at discarding emotions and memories, whereas they earlier during the Kaiju War were simply looking for any pair of candidates that would fit together in a Jaeger. 

It's also likely that drift tech has improved drastically over the years of the war (every other relevant tech appears to have), so perhaps it might simply be better at filtering out data transferred between pilots, or perhaps what is needed to pilot a Jaeger is simply less over the years, as some sort of AI and Jaeger design has improved.