What are the evolutionary benefits of mating for life, as opposed to serial monogamy?

I have a sapient (alien) species that behaves more or less similarly to humans, and that includes their romantic interactions. However, although they court/date each other much like we do, there is a certain mechanic that shapes their cultural understanding of romance. If a courting pair of these creatures lock gazes and share a warm fuzzy "love struck" moment, as we might describe it, their pheromone production spikes, and the ensuing surge of endorphins is so strong that it rewires their brains permanently, devoting the two individuals to each other for the rest of their lives. Pairs who have had this bonding moment have zero interest in infidelity and simply can't fathom getting a divorce, no matter how much they might argue. Non-bonded pairs exist too, but they are much rarer than bonded pairs and are generally either judged or pitied by the rest of society.

The question is, what external stimuli could cause this soulmate mechanic to be favored by evolution? Right now it seems to me like it only serves to make individuals grieve all the harder when their mates die, because they can't "move on" the way humans can. Any suggestions?

EDIT: Further details about the species:

• The bonding process takes several seconds, and the individuals have some control over whether they want to allow it to complete. Often they're too swept up in the sensation to care, but it's possible to "snap out of it" if they recognize the sensation early enough and don't feel ready to commit.
• The bonding moment typically happens after the couple has either been mutually crushing on each other or outright courting/dating each other for a while. There are rare cases of it happening at first sight, but these cases are usually one-sided and are considered to be foolish mistakes.
• If one of these creatures loses their mate, either to early death or because they made the mistake of a one-sided bonding, they are capable of mating again with another person...they just typically don't want to, because the thought of betraying their mate would make them quite uncomfortable. Their biological urges to have sex and/or children may be strong enough to nudge them into taking a new mate anyway, but they'd most likely feel unsatisfied or even guilty.

posted over 8 years ago

Fuzzlewuzzlekins‭

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