From emoji to the queen of hearts in a card game. The symbol for the heart is perhaps the most widely used and recognizable image in the world.
Our emotions affect our heartbeat. An example is that when experiencing tension, the heart beats faster.
Perhaps from that thought the symbolic meaning of heart arose.
In any case, that meaning of symbolizing love has been in vogue for centuries.
Statements in which the heart plays a role also often have a romantic background.