Adult friendships are harder to maintain than younger friendships because life provides less natural structure (shared schools, workplaces, social groups). Without deliberate structure, even close friendships fade through inertia.
What 'structure' looks like
Recurring monthly coffee or call. Weekly text exchange about specific shared interest. Quarterly dinner that's calendared months ahead. The specific format matters less than the recurring commitment.
Why vague 'let's catch up' fails
Without a date and structure, vague commitments dissolve into nothing. Both people assume the other will follow through; neither does. Months pass; the friendship cools.
Pick the 3-5 friends you most value. Set up some structure for each. Energy invested this way produces friendships that last decades.