The piece missing

For months I am wondering what makes we love someone in a romantic way? Can you make yourself fall in love with someone? Why spending a really good time with someone is not enough? Why being comfortable with that person is also not enough? Why considering the person good looking, hot, nice, cool is not enough? What is missing? I thought I could trigger myself to fall in love. I thought I did that once in my life, but maybe there the piece I am missing in the whole story was part of it. Do you know what makes you in love?