This is just another take on a point which was already presented in several other reviews here. The drama is listed as "fake marriage evolving into true marriage" and "hate relationship into love relationship". The problem with that, however, is that the ML and FL simply aren't compatible with each other. This was actually the first drama where I honestly wished the main heroes won't end up together and will go their separate ways. Generally the blame for that (here as well as in Youtube comments) goes to the FL, but in my opinion the the ML isn't without a fault either, and in the end it's really just a natural conflict between two people with different stances and expectations.
The ML is in love with the FL from the very beginning (aka there was no "hate phase" for him), and as a result he turns somewhat overpossessive and ovejealous in the relationship. Conversely, the FL seems to never reach the true "love phase" - most of the time, she takes the ML at best as her buddy/good friend, who certainly has a place in her life, but is barely worth a compromise (not to mention sacrifice) from her side. Good pairing for a drama, just not one with the "love" adjective in it.