People come and go.

Work mates pursue their careers in a different companies.

Colleagues become strangers over time.

Friendships become just memories.

Family becomes more distant relatives over time.

All of this takes place when both sides are not actively working on maintaining those relations.

It takes two to forge any type of this bonds.

So if you value someone and didn't talked with this person for a longer time just use any form of modern communication and have a little chit chat or schedule some hanging out with them. Don't become strangers or you will eventually become alone and lonely.