because that's just how life is. you see someone and almost instantly you want your life to revolve around that person. you see your future in their eyes and you find yourself in their smile. everything about them makes you happy. no one knows why. it just happens.
they fall in love because they find someone they cant go threw life without and they want to keep that person close to them cause they are something special to them and have made a huge impact in there life in a way