The best atmosphere where solidarity can rise and grow is family.
New siblings rise that atmosphere. I would name that solidarity as love and affection.
And here I include parents, who will love each other and the rest of their family in a nice way. I could include grandparents, even more now in our western society, where the pyramids of growing tend to change in the opposite way: more bulk when we’re ascending in that pyramid, since fewer children are born.
We have just to put that pyramid upside down, with the help of governments.
Otherwise, and amongst other reasons, because there will not be enough people to maintain that number of old people. / Photo from: www 1sw org uk