People derive dopamine from many sources (food, games, funny stories, etc.), and in this particular case, sexual content. When two people fall in love, they experience a flood of dopamine which lasts six-seven months, depending on various factors (stress, cohabitation, etc.)

Physical contact between two such people can cause the formation of oxytocin bonds between the two of them, with oxytocin being a neuropeptide (a chain of amino acids that functions as a neurotransmitter). Oxytocin is released during physical contact (but especially sex), and during breastfeeding/childbirth. It leads to strong bonds between lovers and between parents and children.

It's been suggested by some marriage counselors and neuroscientists that sexually derived novelty dopamine (i.e. dopamine with someone WITHOUT an oxytocin bond) can diminish oxytocin bonds.
From the words of some of them, that's why the best cure for breakup pains (caused by oxytocin) is to go out and love someone new, and why the people who cheat (and therefore allegedly reduce oxytocin bonding) tend to have emotional dysregulation towards their partner, even if they don't get caught.

Oxytocin has also been the "monogamy hormone", for both these alleged qualities and how it bonds the people who receive dopamine together.

However:
There's no limits to how many oxytocin bonds with lovers a person can have, and sexually-induced dopamine doesn't actually affect those bonds whatsoever.

Change my mind.