These days the concept of love has changed. People tend to move from one person to other. They leave once the "spark" dies.
When something is new it has a charm to it. Just like when a baby is born we adore him/her and the joy is overwhelming. But once he/she grows old, he/she becomes a part of the family, a routine. But we still keep loving the child. Even when the new joy of them being born has worn out. It's because we love the person.

The same is in the case of a relationship. The spark will not remain there forever. But the love will. And if it doesn't then it wasn't love at all. It was just lust.
Sex is a part of a healthy relationship. And should be spiced up from Time to time. But it is not the only thing in a relationship. A healthy relationship requires everything :- passion, care, love and most importantly personal space.