I really not know when we started celebrating birthdays but it clearly shows our attitude of giving importance to every individual. Which clearly shows our natural quest for celebrating individuality of person.
It may possible that we started celebrating birthdays unknowingly but looking deeper into it clearly shows us the mystery behind it. Because on contrary to plants and animals which exist in species we exist as individuals! Like, when we say “Tiger ” then we hardly mean to any specific tiger but we say it in general sense. Means here we accept animals and plants as species. Even biologically it is correct because plant and animals still not evolved to the level of achieving individuality. Here we are little different! Like if we say that these people belong to same country, let’s say Germany. But that doesn’t mean these all people are of same character! They may be completely different to each other. Like Adolf Hitler and Albert Einstein were belongs to same country Germany but one was cruel dictator and other was peace loving scientist !! That’s beauty of human nature we are only species to achieve this characteristics of individuality.
Celebrating anything bring joy in life. When we celebrate someone’s birth we come closer to life because it is like dancing with rhythm of nature and like rejoicing beauty of universe. Every times we celebrate someone birth we celebrate possibility of his achieving individuality and hence density in life. This may be one of the reason we remember birthdays of great people. Because by reminding them we salute their quest and achievements . And it also remind us that how they live their life with their own terms and blossoms like flower. Whether we celebrate birthdays of great people or of someone whom no one knows we all do it for only one reason and that is to respect and show gratitude to their “individuality “. Because deeper we all knows every next birthday is new opportunity to come closer to nature and to understand oneself more clearly and hence finally discovering once destiny.
Post Script : When I was in villages I was completely unaware about the concept of birthday celebration! People in villages hardly care about birth dates! They seems too busy completing end meets. It was in my Navodaya days I first time encounter the custom of birthday celebration. Then after I start finding reason behind this celebration other than mere accidental birth date! And for my surprise the whole custom seems to concern more to once individuality rather than once birth date and that was marvelous finding.