We can only track so many people, relationships decay, networks shrink with age, modern life multiplies the noise, and we're wired for the short term.
The idea
If you've lost touch with people you liked, that's the human condition at work. Five forces push against every relationship, and knowing them tells you what a system has to counteract.
Limited capacity. Robin Dunbar's research puts our natural limit at roughly 150 relationships, in layers of about 5, 15, 50 and 150. His 2001 study of British Christmas cards found an average network of about 154.
Decay. Memories fade with time and get crowded out by new ones (time-decay and interference theory). Goodwill fades with them. Every relationship is on a down escalator.
Age. Networks grow through our teens and twenties, then shrink. How many friends does your mother have?
Volume. The internet means more people, more channels and more competitors reaching the same people you are.
Short-term wiring. Hyperbolic discounting: most of us take $5 today over $100 in a year. Relationships pay off on long, unpredictable timelines.
How to do it
Stop feeling deficient about the people who drifted. Nobody is to blame for the default.
Unless you can recall hundreds of people in detail, lean on technology, process and discipline: a place to keep what you learn about people, and steady interactions so mindshare never decays below a threshold.
Protect time for the long-term work, because the short-term work will always ask for it first.
Example
As a freelancer I wanted to build my own software product and had the time to do it. Week after week it didn't happen, because clients paid now and the product paid someday. Contactually would have stayed an entry in Evernote and a prototype if I hadn't carved discipline into my schedule to build it. It's no different with our connections.
Caveperson Wiring: Our brains evolved to treat outreach as danger and to forget people, so go easy on yourself, then build systems that compensate.
Mindshare Decay: After you meet someone, the odds they think of you slide toward zero unless periodic, valuable touchpoints push the line back up.
Networks Shrink With Age: Networks grow through young adulthood and then shrink, so intentional effort has to replace the circumstances that used to keep people close.
Time and Fear, the Two Universal Blockers: Almost every reason people don't stay in touch comes down to two things: not having time, or fear of how the outreach will land.