That phase at the beginning of a relationship where the feeling component of Love is strongest and where conflicts are few. I think this is mainly driven by novelty and getting to learn much about a person in a short period of time rather than incrementally as people grow. Maybe there’s a delusory aspect here as well, where when you haven’t seen everything around a person, its easy for your brain to fill in the gaps and romanticize everything about them.
Eventually, like all feelings, your emotions regress to the mean which kicks off the Love cycle which is why love is not purely a feeling. Once you get used to certain things, you start to loose appreciation and regard your relationship state as something natural (Taken for Granted).
A Take (ET)
A take on relationships that fail soon after the honeymoon phase, is that this is caused by putting your significant other on a pedestal during that honeymoon phase and coming to realize that they are just a person the more you get to know them, with their own flaws and shortcomings.
My reflection on this take is that since this is a point of failure stemming from disappointment (or putting someone on a pedestal), this is a point of failure experienced by those who heavily rely on their partner for support (in some sense of the word).
It is unrealistic to expect to be persistently in a honeymoon phase during a long-term partnership. At the end of the day, half of love is action.