Friends come and go. People move away, change jobs, new people move into your town, city. Some help you out in bad times then move on. Others stick around but are busy with their lives and you catch up occasionally over a drink, a meal or perhaps a Christmas.
It's complex because we move around such a lot now, it's unusual to have friends for life but even short friendships can be real and meaningful. You learn something from them and hopefully they learn something from you.