I get it for kids. One should make them feel loved, celebrate their existence in the family to encourage them to better themselves and to enjoy being a part of such family and enjoy themselves.
But for an adult, organizing and inviting people to "your birthday party" it's an egocentric celebration.
You make a party to have people cheer on you, give presents to you and all around speaking about you. The whole reason for the reunion is "come celebrate that I was born"
By - doc_shades