Hancock just fell in love with luffy because he is a one of a kind person. Anyways Oda can do what he wants with his charcters, its not like they are real.
Rohdes: Watch ep 415 & 416. The answer lies in them. When hancock asks luffy to make a hard choice between 2 alternatives the choice which luffy decides surprises her. Then Elder Nyon explains what luffy did to a tenrubito & she just gets more interested in him & is convinced by Elder Nyon to reveal her past to luffy. After revealing her past to him the words which luffy says are enough to convince her he is the person who is right for her.