Last Valentine’s Day represents the experience and aftermath of a breakup as a time loop in which the PC relives the last day of the relationship over and over, passing from shock and disbelief through despair before finally reaching the point where he’s able to move on with his life. The world around him reflects his mental state—the weather, the condition of the park he passes through, and the lives of the people around him go from pleasant to miserable, then gradually improve again.
This externalization of the PC’s feelings serves as somewhat of a substitute for actual interiority—there's little specific detail to be found here, so I don’t have a strong grasp of who the PC is, who his partner was, or why their relationship fell apart. The most we get is a letter from the ex describing their relationship as "like a roller coaster," which, in addition to being a cliche, has a whole range of possible meanings, some of which would make the PC a rather unsympathetic figure. But the evocative descriptions of the environment and the predicaments of the somewhat more distinctively drawn side characters help to ensure that the game sounds the emotional notes that it means to, for the most part.
The game effectively captures the post-breakup emotional arc of a person who has been dumped; choosing to represent this as a Groundhog Day loop emphasizes how difficult it can be to move past this experience, and the fact that choices don’t matter much makes sense inasmuch as this kind of post-relationship grief is, to a degree, something you have to just wait out. (Others have suggested that this passage-of-time aspect makes the time loop framing a bad fit, but to me the emotional logic of it made sense—the PC is obsessively retreading the breakup in his mind, but with each cycle he comes a little closer to being able to actually put it behind him.) But without any distinctive characterization for the main ex-couple or insight into how things got to this point, it all feels a little hollow in the end.