The Rice University Neologisms Database

To like or adore. Although to heart someone means to like or adore them, the verb heart doesn’t carry the same serious meaning as does the verb love. For example, heart can be used among casual friends to express affection for one another. The physical heart is generally thought of as the source of human emotion, especially the emotion of love. Occasionally the phrase “I love you” is written with the word love replaced with an image of a heart. This usage of heart as a verb comes from reading this image literally as the word heart instead of taking it as a symbol for love. Therefore, this use of heart is an example of metonymy.