The poet compares the ups and downs of love to the highs and lows of a roller coaster. In a healthy relationship, both partners will share these exhilarating moments of love and romance which is symbolised by the thrill in this ride
But the twist occurs as the metaphorical ride progresses. Sometimes, there will be a point where one person begins to experience a different set of emotions. They begin to feel disconnected.
And at that very moment, one starts doubting his partner and their decisions start becoming influenced by those doubts that eventually end them falling off (ending the relationship) which the poet says "I did, but I didn't want to. And that's the cruel and painful thing about letting go of someone's hand."
Therefore, this powerful metaphor of a roller coaster captures many different dimensions of emotions and experiences common within a typical, yet complex romantic relationship.