Why would people kneel whenever they make a marriage proposal?

Native speaker’s answer
Rebecca
Kneeling when one proposes is a tradition that comes from when knights would drop to one knee out of respect for royalty and noblewomen. Many medieval paintings depicting love show men kneeling before a woman. The practice has stayed around since then but has been adapted mainly for marriage proposals. I suppose it still shows respect and devotion to the other person, a kind of devotion.