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
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Retiring flight numbers after crash is mainly done to prevent the flight evoking negative emotions among future passengers.
You don't want the flight number to conjure up images of crash while booking tickets, especially in when you type the flight number and google shows up the wreckage just below the flight data.
Also, it would be quite insensitive, with friends and relatives being reminded of their loved ones every time they hear about the flight.
However, it is not necessary that the airlines have to retire the flight numbers because of a crash.
A number of them are being used, a few examples being: Air Canada Flight 797 Delta Airlines Flight 723 Air Asiana Flight 214
aviation.stackexchange.com/users/9731/aeroalias
You answered your own question, they retire numbers because of the emotional context.
Some people think that using such a number again shows a lack of sensitivity to the dead and their families, others might think that getting on a flight with the same number would be bad luck.
It's irrational, but many people would feel uncomfortable getting on a flight with the same number.
aviation.stackexchange.com/users/1791/GdD
It's also done for calming passenger superstitions about such things as well.
Kind of the same reason that many skyscrapers do not have or list a 13th floor or nobody would board a flight listed as "United 666" or that left handed people are possessed by demons, etc.
You would like to believe modern people would not be influenced by such nonsense, yet how many people did you see reading horoscopes on the train on your way to work this morning?
aviation.stackexchange.com/users/15637/Carlo Felicione
I am a retired airline employee, the reason for retiring a flight number is for the respect of any fatalities.
No other secret reason.
(At least the “original” Airlines)
it was ALL ABOUT RESPECT AND NOTHING ELSE.
aviation.stackexchange.com/users/40682/Katie
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?
Why do airlines retire the flight number after a crash?
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<airline-operations><accidents><airlines>
aviation.stackexchange.com/questions/33766
aviation.stackexchange.com/users/18218/clueless007
Flight numbers are usually retired after a crash out of respect for the dead and also because passengers in the future may be reluctant to take a flight with tha same number as a crashed plane. There are a few examples of where flight number are still being used after a crash, for example Air Canada Flight 797  Delta Airlines Flight 723  Air Asiana Flight 214 . The main reason flight numbers are retired is out of respect for the victims and their relatives, in order to calm the superstitions of the passengers and to avoid the negative context that inevitably comes with a crashed flight. Also, they are often retired to avoid confusion when looking up the flight number. However, there are some flight numbers of crushed flights still in use.
They are often retired to avoid confusion when looking up the flight number. The main reason flight numbers are retired is out of respect for the victims and their relatives, in order to calm the superstitions of the passengers and to avoid the negative context that inevitably comes with a crashed flight. There are some flight numbers of crushed flights still in use.