Answer by Rohan Golwalkar:
It has more to do with the emotional connect with the movie.
Movie buffs like book lovers are invested in what they do: hence their opinions can be rigid and hence they defend it hard.
Like Christopher rightly mentioned Art is subjective.
Adding to this: may be rather than finding out why criticism is taken personally we can probably engage them by asking question differently:
Example: What were the aspects of Birdman (insert any movie of your choice) which earned it so much praise.
Try it out may be you might end up being good friend with the movie buffs.