The area of the striatum that is activated by sexual desire is also activated by any actions that are inherently pleasurable.  For example, the same area of the striatum is stimulated when a person eats food and when a person has sex.

Meanwhile, the area of the brain activated by feelings of romantic love is also responsible for assigning value to pleasurable activities.  So this part of the brain assigns a high value to sexual pleasure, and can eventually morph the pleasurable feelings into feelings of romantic love. 

Source: Science Daily