One thing I've noticed when things shift in a relationship is that the guy never admits it.

Sometimes a few months into a relationship, I will get a gut feeling that something is off - maybe he doesn't seem as enthusiastic over text, maybe he stopped giving as many compliments, maybe he just seems less present during conversations, and then I'll ask what's wrong and they will tell me that everything is fine... only for them to continue putting in less effort, act less present, and give less compliments. Obviously that's when the relationship ends, but I just want to know why they wouldn't just tell me.

Why won't they admit that they aren't as into you anymore and just end things?