Truth or Dare and trust
If young people build trust, it is through shared rites of passage. Some of these activities have existed for ages, although new ones seem to get introduced during the apocalyptic second half of a violence wave, when you really need to count on others. The last such time was the mid-1970s through the early '90s. Of that period's cultural inventions, my bet for which will prove to be an enduring trust-building rite is the game Truth or Dare.
Games involving skill exaggerate some pecking order, which isn't bad because preserving social stability sometimes requires us to be reminded of our place. But occasionally, and especially during rites of passage, we must experience a leveling of ranks. Otherwise, group members won't develop strong bounds. Truth or Dare is ideal, involving no skill, no teams, and no score.
If you didn't trust the others enough to not take advantage of you, you'd never play in the first place. Joining in signals that you trust them enough not to ask a relationship-shattering question, or subject you to a degrading stunt.
Each time a person's question or dare to another is provocative enough to keep the game fun, but doesn't go so far as to humiliate, you grow more assured of their trustworthiness. They held the reputational fate of another in their hands and chose not to abuse it. This is an even stronger signal given that they're not fully socialized, and can find the temptation to royally embarrass someone overpowering.
Sometimes, however, you do meet a selfish or fair-weather friend who crosses the line. Even so, it's better to discover who to keep at arm's length from playing some silly kid's game, rather than from getting stabbed in the back for real.
Then there's the cross-sex appeal. Girls like the gossipy Truth side, while boys are drawn to the show-off-iness of Dare. Peer pressure prevents them from always choosing one or the other, though. This negative feedback loop is typical of healthy boy-girl interactions -- girls are kept from spiraling out of control in the gossiping direction, and boys in the Jackass direction.
And if while growing up boys and girls don't play these trust-building games, they'll get the shallow and retarded relationships of Millennials -- mostly avoidant or non-existent, though also transient hook-ups and relationships of convenience where they're only sexless acquaintances, rather than two people who complete each other.