People take anti-depressants usually if they have depression not because they felt unhappy at the moment due to sad event (have death or illness in the family for example).

If a person suffers from depression, then it's not as simple to just feel happy. And not everyone who felt unhappy about something needed to take anti-depressants.

Typically if you don't suffer from persistent depression your feeling of unhappiness (due to something specifically sad happening) will eventually subside and you are able to cope with what happened without falling into depression and needing meds.