I suppose it comes down to whether you decide you want to like it or not. If you want to like it, you focus more on why you like it; if you don’t, you focus more on why you don’t. You can certainly discuss flaws in something you like, but primarily after it’s been established that the thing you like is the thing they like. Kind of like picking on a mutual friend behind their back, as opposed to listening to someone who hates their guts.
It’s not even really that. It’s just nitpicking I can’t stand. Yeah, you can nitpick and find seeming logic flaws in the plot or whatever. But it’s just as easy to find reasonable excuses for them. For me, these kind of little things ultimately don’t matter in regards to the movie’s quality.