About your the wording of your question:
Why do English people... ["do" not "does"]
Many years ago most English speakers were illiterate (ie., they could not read or write).
So when they signed a document, such as a legal document or a letter, they would use an X to represent their signature. The X also had a religious meaning (I don't know what it is).
So when they finished signing the document they would kiss the X, the same way they would kiss their holy book to show respect and truthfulness.
Years later, the X became a way of showing or sending a kiss to the person who was receiving the letter.
Nowadays, in romantic letters, we often use alterating X's and O's (letter O) after we write our signature.
For example,
XOXOX
X = kiss
O = hug