People unite facing the common enemy
When someone is taking out the gun, starts shooting at you and everyone around you, there is only one thing you can do to survive. Unite with everyone around you. It happens in seconds. It's survival instinct.
It may sound pretty obvious. But how about bringing that knowledge to a wider angle. How else could people unite? When getting married? Joining for a business? Meeting with friends?
What's the goal of those alliances? Be together? Have fun? Make kids? Build families and companies?
What's the difference between those reasons to join and when joinings against the common enemy?
Time and certainty. It may take a lot of time and can end before the goals are reached.<