The answer to this is very simple.. it's the human tendency of never being satisfied, and always asking and hoping for more.

This is not just about love, but take an example of anything in your life.

When you are a kid, you are taught that once you pass your 10th std, you wouldn't have to struggle so much.

Well, the reality changes when you realise this is just the start.

When you clear your MBA entrance exams, you are proud of yourself but you soon realise the bigger obstacle is getting a good paying job after your MBA.

What seems like a distant dream, when becomes a reality, you change your dream.

In the context of love, you may find 5 traits in a guy that you think make him THE one, today.

Tomorrow those 5 traits become a basic necessity, and your aspirations grow.

So it's really not about falling out of love, it's about being Content and happy with what you have.

If you manage to do that, no-one will fall out of love :)

~S