People kiss at weddings because kisses were used in ancient times to seal contracts

In ancient Rome, when people reached an agreement, they kissed to legally seal the contract. This practice extended to marriages as well (which were a form of contracts) and it has remained until today.

Interestingly, you know how Xs are used to signify kisses when you text? This also has to do with contracts! In medieval times, there were a lot of illiterate people. Whenever they needed to sign a contract, they would write an X on the paper, then kiss it as a way to signify that they've sealed the contract!

(Source)