Some couples may disagree, but romantic love lasts little more than a year, Italian scientists believe.
The raised levels of the protein tailed off after a year or so
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.
The team analysed alterations in proteins known as neurotrophins in the bloodstreams of men and women aged 18 to 31, the Psychoneuroendocrinology journal reported.
They looked at 58 people who had recently started a relationship and compared the protein levels in the same number of people in long-term relationships and single people.
In those who had just started a relationship, levels of a protein called nerve growth factors, which causes tell-tale signs such as sweaty palms and the butterflies, were significantly higher.
Of the 39 people who were still in the same new relationship after a year, the levels of NGF had been reduced to normal levels.
Report co-author Piergluigi Politi said the findings did not mean people were no longer in love, just that it was not such an "acute love".
"The love became more stable. Romantic love seemed to have ended."
And he added the report suggested the change in love was down to NGF.
"Our current knowledge of the neurobiology of romantic love remains scanty.
"But it seems from this study biochemical mechanisms could be involved in the mood changes that occur from the early stage of love to when the relationship becomes more established."
However, he said further research was needed.
Dr Lance Workman, head of psychology at Bath Spa University, said: "Research has suggested that romantic love fades after a few years and becomes companionate love and it seems certain biological factors play a role.
"But while we are a pair-bonding species, there is some doubt over whether this is within monogamous relationships or not.
"Different societies have different practices and trends."