Heart used to symbolize health and life.
A common bit of widely understood symbolism in Video Games: a heart is used to represent someone's health or life. The Life Meter or lives counter may be made of hearts, and health-related pickups may be in the shape of hearts as well (see Heart Container). The reason for using the heart here is because of its positive connotations: "lifeblood", love, as well as it being considered the most important organ for a long time (nowadays, the "vital organ" is thought to be the brain, but brains are rather squicky and have different, intellectual connotations entirely). See Heart Symbol.