Naturally, the best friendships survive when both parties proactively seek to enhance and mature their relationships. They anticipate or sometimes intervene in crises. They periodically call or write or talk. They know to send reminders that they value the friendship. But people have different capacities, resources, and awarenesses for being proactive.
Some people are better at being opportunistic friends.
When the occasion presents itself, they rise to the occasion and pitch in with some project or with the consoling pat on the shoulder. They may not keep abreast of events in your life but they're there when they can be, showing that they do care. The little things that friends do for each other remind us that we are never truly alone and that there are people who value us for who we are. Friendships can suffer from distance, busy schedules, work, and other important things. Sometimes, in spite of the effort, a friendship fails to survive a crisis.