When you massage someone, the levels of oxytocin go up in the brain, and oxytocin is one of the chemicals that drives attachment.

Any kind of novelty or excitement drives up dopamine in the brain, and dopamine is associated with romantic love.

When you fight, anger drives up testosterone in both men and women.

Blushing is thought to be linked to increased levels of norepinephrine in the brain, which may be associated with romantic feelings. It signals that we are interested and excited, which is attractive to men.

Romantic love allows you to focus mating energy. Attachment sustains that relationship as long as necessary to raise your baby.

The human brain is built to compare; it's Darwinian to consider an alternative when one presents itself.