Italian scientists (and really, what else would they study?) have proved it:

Some couples may disagree, but romantic love lasts little more than a year, Italian scientists believe.

The University of Pavia found a brain chemical was likely to be responsible for the first flush of love.

Researchers said raised levels of a protein was linked to feelings of euphoria and dependence experienced at the start of a relationship.

But after studying people in long and short relationships and single people, they found the levels receded in time.