People leaving your company or team

When people you know, and consider as great employees want to leave your organization or your team it is an event that you should never under-estimate. As always, somebody can leave the organization for private reasons, or professional reasons. Let's talk about professional reasons.
He/she can leave because:

• he/she is not happy
• because he found a better job
• he did progress in its job and maturity and move forward 
• he/she had a very good opportunity.
• Because he will get a better salary

The event that a colleague leave the company is generally triggering in us millions of questions.
Why:

• this guy had a very good opportunity, the question it raises is why not me?
• he/she left because the working conditions are not good enough ... why do I have to tolerate them myself?
• why can he get more money and not me?
• How do I feel working in this company?

The more events you trigger in a time interval, the more disturbing events you will send to people in your team. Then what? Thinking about its position, where you are in your job and where you want to go is not bad per se. It can just open the door for a massive internal questioning.

Anyway, it is very annoying to receive a farewell email message saying: "Everything was great here, but I found something greater" ... and my little internal voice to conclude "and not me".