it's very simple, in fact, i feel i could paint the whole picture for you with zero effort
here's how their life probably looks like
- they live in the same neighborhood or have both lived in the same place for a long time, no moving around
- the two of them are somewhat poorly socialized and mostly socialize through neighborhood and family
- most people in their neighborhood are happy and sociable, and their parents are happy and sociable and have many family friends
- both of them are often outside and involved in some of these private family friend events, without putting in much effort, just accepting invites
- once they get together, their families create additional incentive for them to stay together, for example, the girlfriend becomes friends with the boyfriend's sister, adding an additional incentive for the couple to stay together, because if they didn't, it would break the new friendship between the two girls
- they stay together as their relationship is essentialy formed, and even sustained, by their environment, more than by their own actions
hope this makes sense to you too bro