Philosopher Massimo Pigliucci explains why it is on the Huffington Post:

Why, exactly, is it that broadly speaking we condemn the practice of “trading up” one’s partner, as we do, say, with cars or smartphones? After all, if I love someone because of certain characteristics of that person — say, her beauty, her wit, her intelligence, and so on, it is perfectly possible that, eventually, I will meet someone with those some characteristics “plus,” a better version of my current companion. Why, then, should I not exchange the old model for the new one, so to speak?

For a variety of reasons, answers the philosopher. For one thing, the more time you spend with someone the more unique experiences and memories you accumulate, which means that the relationship is growing in an important sense, a sense that would be nullified if you were to change partner and start anew. But a better reason is provided by Aristotle in the context of his so-called virtue ethics: trading up objectifies a companion, thereby undermining her human dignity. And consistently doing so results not only in developing an awful reputation among other human beings, but corrupts our own character, making us worse people. Conversely, practicing virtue is the path to what the ancients called eudaimonia, the happy (because moral) life.

I’m not sure I’m entirely convinced that trading up is always wrong. 

I think it’s obvious that “trading up” in love can absolutely be justified. For example, if I am in a relationship that is bringing me misery, I would have good reason to search for someone who is a better fit. But that’s not what Pigliucci is talking about. Pigliucci is addressing the person who is in love, and in a relationship, with someone else, but who is still looking for a “plus” version of their current significant other.

Yet while this might not be proper, is it really immoral? If the plus-seeker is performing this search while in a relationship, I think the answer is yes. In this case, the plus-seeker would be misleading his or her significant other.

But what if the plus-seeker broke up with his or her significant other — or at least made said significant other aware that they think someone else might make them happier — then performed the search for the plus? Is there anything really wrong with that? In this case, there would be no intentional harm; no lying or misleading; and both parties, in the end, might get what they want (i.e., better partners). So where’s the problem?