Leo is the astrological sign that is said to rule the heart, and those born under this sign are said to be the most passionate people of all, which is a good symbolic fit for this sign's representative, the lion. In addition to ruling the heart, Leo is also said to rule the spine, though that anatomical feature may not have the characteristically apparent features that align with what are said to be hallmarks of the Leo's fiery, intense personality. As the sign that is ruled by the heart, it can be said that Leos are unhappy if they are not in love or romantically satisfied.
A Leo's passion is said to translate to great loyalty. Those who are born under this sign may be particularly committed to making romantic relationships last, though that doesn't necessarily mean that a Leo will stick around if he or she has been betrayed. Part of a Leo's passion is said to include a dominant proud streak, with this pride likely influencing the Leo to refuse to tolerate or forgive broken trust. Like the lion, the Leo is proud, and though matters of the heart are said to be important to this sign, pride may be the Leo's dominant trait.