There are endless ways to say "I love you." According to the five love languages, for some people, doing chores for your partner is an adequate way (acts of service). But we verbally inclined individuals want words — and cinema has provided us with a litany of options for "I love you." There are romantic quotes best deployed for tortured and forbidden loves. There are quotes for exuberant, easy love.
Essentially, there's a quote for almost kind of love because there's a movie about almost every kind of love. The movies give us a chance to experience all of these loves without actually having to live through them. You wouldn't want to live through Titanic, but you could watch the movie so many times that you accidentally memorize the speeches.