They both saw the end before it even began. Willow - a self centred and purposeful girl - meets Marcus - outgoing boy and the king of partying - at the library. They instantly felt something like a spark between each other, but after a while they realise that the spark they have cannot be turned into a light.
Not only the age, but also their life goals are really different.
Willow has to make a decision - career or her soulmate. At the end, something happened and it made her decision easier.