Why am i not interested in a girl unless i fall in love with her instantly?

Whenever i meet a girl the 1st thing i ask myself is could i fall in love with this person? invariably the answer seems to be no. i fell instantly in love with an ex-girlfriend some years ago and ever since then if i dont get the same instant feeling from someone i back off. is love something that can grow in time? i seem to tell my self if the feeling that this person might be loveable isnt there right away, then it never will be at all?