Why do airlines retire the flight number after a crash? Perhaps it seems obvious, but I couldn't find a plausible reason why they do so other than that it evokes negative emotions among passengers and crew. Could there be other reasons to do so?

For example, flight number MH370 was retired as a "mark of respect" for the passengers and crew. 

And out of interest, is there any case where an airline company reused a retired number for any reason or in certain circumstances?
consider if you Google a flight number you can see the flight status, imagine if the number was from a past crash, you might for a moment think the one you're looking up had crashed