"Life is a roller coaster" is a metaphor, which is a way of equating one person, place, or thing, or idea with another person, place, or thing, or idea to show that some attribute or attributes of one is or are similar to some attribute or attributes of the other. What is of most importance is for the listener or reader to be able to identify what attribute or attributes are being equated. Language is filled with metaphors because they are such an important way of communicating meaning. In order for us to understand something, we need to have a concrete, sensory image in our minds, and the metaphor is what provides this.
Let's consider "Life is a roller coaster." The person who says this does not mean to say that we are all strapped into little boxes at a carnival. What attribute is meant to be equated? It is that life has many ups and downs, as a roller coaster takes us through many ups and downs. In the movie Parenthood, the grandmother says that some people are on a roller coaster, while other people are on a merry-go-round throughout their lives. Her meaning there is that some people are more adventuresome, taking a chance on the ups and downs, while others are risk-averse, and simply go around and around on the same track.
Interestingly enough, the idea that life has many ups and downs is a metaphor, too. As in the roller coaster metaphor, the idea of ups and downs is not literal as we apply it to our lives. When something goes wrong, we have not decreased our elevation, and when something goes well, we have not literally increased our elevation. We use the very same metaphor to explain our emotional states. When we are happy, we say we are "up." When we are unhappy, we say we are "down." There is actually a logical explanation for this, the reason that we use ups and downs as a metaphor for good times and bad. If you look at any person who is unhappy, he or she is likely to be slumped over, while someone who is happy seems to stand straight and tall.
According to linguists, all languages use metaphor. It seems to be a human need, to communicate our ideas concretely. Once you become aware of what a metaphor is, you will see that the world is filled with them. When we speak of education, we speak of building a "foundation" for learning. A foundation is what a building is built upon, so that is a metaphor. In business, we discuss "growing" a business as though it were a garden. They are a staple of poetry and songs, as Donne says, "No man is an island" or Simon and Garfunkel sing, "I am a rock." In literature, they are everywhere, and one of my favorites is from The Great Gatsby, "And so we beat on, boats against the current, borne back ceaselessly into the past" (Fitzgerald 189). I hope this opens your eyes at least a little to the amazing and wonderful world of metaphor.