The first reason is of course the passion one feels for the other. Call it hormones, mind, heart, emotions or whatever, one falls in love with another person and cannot live without him/her. Physical beauty is not the sole criteria in romantic passionate love. A very beautiful girl can passionately fall in love with an ordinary looking man to the surprise of all. There is something more that is yet to be explored. Ask a lover about why he/she loves her beloved and you will never get a logical answer. Love does not measure qualities, nor sets any standards. Love happens.
Sometimes the love arises out of appreciation of qualities of the other. This love is different than romantic love, but can be equally magnetic. You may get attracted to some persons singing so much that you want to be with him/her at all times. It can be any quality, but it is the appreciation of quality that brings forth this love. In this case the love is passionate love with that quality.
In some cases two persons find so many things alike between each other that they believe they are soul mates. Made for each other, these lovers are bound by the common qualities. What ever may be the reason for falling in love, please fall in love because it is incomparable.