Melding
Had plenty of reason lately to consider this topic. Second-hand observations at close range lead me to a new appreciation of why (generally) marriage is easier in certain respects when entered into while young.
The older one gets the harder it becomes to adapt to the presence of another person. We grow ossified in our patterns and preferences, and defensive about changes to them. When young we are still determining who we are so it is far simpler to adapt ourselves to the person we are with, and visa versa.
Once we cross the crotchety threshold adaptation is increasingly harder and less likely. Finding a partner at this point means locating someone with a compatible set of sharp edges that will fit ours. Not so much molding and melding goes on so we better find the right fit from the begging or those nasty corners on our personalities could cause some harm.