Those who have spent much time interacting with Millennials have noticed how withdrawn they are. The average member won't initiate anything, whether social (getting to know new people) or mechanical (that bamboo is starting to look gnarly in the back yard, better clear it out).
This has lead casual observers to describe the generation as passive, but that term really means that the person will pitch in and perform various tasks once someone else — the initiator or instigator — has gotten the ball rolling first. They are willing, perhaps even eager to join in an activity — they just can't start it.
Yet Millennials are not only incapable of kicking something off, they fumble the ball once it has been perfectly thrown to them. Beyond being anxious about introducing themselves to new people, they don't know how to respond to someone else introducing themselves first, let alone how to keep the back-and-forth going so that the result is a relationship rather than a mere encounter.
They don't know how to act, but they don't know how to react either. They're just plain awkward, and it keeps them from developing a normal system of relationships.
"Passive" would actually be a better description of the average Gen X-er. As long as there's an instigator around, X-ers are perfectly comfortable joining in the mischief. Or accepting a subordinate role in a hierarchy, under a leader, mentor, or guide. Do you want to go bowling? "If you guys are going, sure." Where do you want to go tonight? "I dunno, I'm cool with whatever." Yeah, me too.
It normally doesn't devolve into the blind leading the blind because despite the majority tendency, there's always at least one leader or instigator in their social circle.
That leaves the Boomers as the assertive ones. There's a lot more playful, half-serious ribbing and joshing among them because they're all trying to assert themselves and have the others in the group be subordinate. They're more willing to be creators, while Gen X prefers to be fans.
You see this clearly in stereotypes about husbands. The stereotypical Boomer husband was cheating on his wife with a secretary or waitress, endangering his marriage to assert his libido. Gen X husbands are more likely than Boomers to see their role as the dopey dad and the henpecked husband, whether they resent that role or are cool with it, y'know, as long as the wife is cool with it.
The stereotypical Millennial husband is neither an assertive nor a passive partner in the marriage. Millennial husbands and wives are more like gender-non-specific housemates who occasionally have genderless sex. None of the household tasks get taken care of because neither is capable of being the leader or the follower in getting them done. Maybe if we both ignore the bamboo jungle in the back yard, it will be nice and just go away to infest some other home.
What underlies these differences seems to be how much of their social development, say ages 5 to 25, took place in an outgoing (1955-1990) vs. a cocooning period (since 1990). Boomers, particularly the later ones, developed entirely within an outgoing climate, which allowed them to reach an adult level of assertiveness.
Gen X developed partly during an outgoing climate, but also during a cocooning climate in early adulthood or even adolescence. That allowed them to mature beyond childish awkwardness, although still retaining more of an adolescent approach of "I'm up for it if you are". That effect is more pronounced among the later births in the generation.
The poor Millennials who grew up entirely in cocooning times, under helicopter parents no less, never even made it to the adolescent stage. Now that they're nearing 30, they realize that they're supposed to be able to take part in the back-and-forth, following either an assertive or passive role, and some of them are making a conscious effort to practice. But at a gut level, their instinct is still to just stand there and go, "OK, so now I guess we, uh.... well, this is awkward..."