Generally it's because he makes you happy, is thoughtful, and considerate of your needs, Most people have their own "shopping list" of what they want in an ideal mate. If they come across someone who has those traits they fall madly in love with them.
On the other hand there are those who (say) they want one type of person but are (drawn) to the complete opposite. They're unhappy but can't imagine life without their mate even if he/she constantly mistreats them.
Maybe the sexual chemistry is "off the charts" or they're in love with their mate's "potential". These people tend to mistake a (crumb) of devotion for the whole cake. Either way they don't love themselves enough to believe they can do better so they refuse to move on.