While proposing on one knee is a practical gesture of respect and homage, its roots likely trace back to centuries-old historical practices or a combination of several. People have been kneeling to show respect or reverence for thousands of years (it stems from the Latin meaning "to bend the knee"). Proper salutations tied to social rank may have originated in the Persian Empire. Historian Herodotus observed in around 430 BC in Iran: "When one is a little more exalted than the other, the kiss is given on the cheek." "Where the difference in rank is great, the inferior prostrates himself on the ground."
This salutation system, known as proskynesis, was adopted by Alexander the Great a century later when he took over the empire, and some historians believe that kneeling was part of this ritual. However, not everyone agreed, as many of Alexander's existing Greek and Macedonian subjects thought such gestures should be reserved for the gods, not a mortal ruler.
Nevertheless, the idea of kneeling as a sign of respect would become popular in both religious and secular contexts in the future. For example, Catholics kneel when facing the Eucharist (consecrated wafers believed to be the body of Jesus). After battles, European knights in the chivalric era would kneel before their commanders, often referred to as lords, who dubbed them knights. In fact, even today, citizens knighted by Queen Elizabeth II are expected to kneel when the dubbing ceremony takes place.
According to Bustle, the romantic connotation of knights bending their knees may have emerged during the golden age of chivalry. In the 11th century, knights began forming close bonds with women at the court, a tradition later known as "courtly love." Since the women were usually already married, the nature of the relationship was often not sexual but always a serious commitment. Knights pledged to serve and honor their beloveds with the same fervor applied to their lords and kings.
While there is no direct connection between this medieval trend and today's tradition of proposing, many artistic depictions of courtly love carry the characteristics of a man kneeling in front of a woman.
In summary, kneeling has long symbolized loyalty and humility, expressing a desire for someone to live with you forever. However, asking the question by lifting both feet does not violate the established etiquette for proposals.