Cupboard love
"Cupboard love" is an English expression referring to affection that is given purely to gain a reward.
The term derives from the way in which a cat will give the person who feeds it superficial "love" (which is not normally exhibited) when it wants to be fed. The phrase is most often applied to human activity, for example when a child will say "I love you so much" before or after requesting a treat, for example an ice cream. Cupboard love is a milder version of conditional love, where love is given solely in response to certain behaviour.
Example in Literature: From page 47 of the children's book Miss Hickory by Carolyn Sherwin Bailey, copyright 1946: "Cow had been gone from the barn all day, and Mr. T. Willard-Brown [a tomcat] was anxious about her. Not on Cow's account. Oh, no! He had only cupboard love for Cow and had missed his daily saucer of warm milk."