Let's face it, entitled people act that way because it works for them enough of the time to be repeated. I think with a lot of entitled people, it probably starts out small, and the people close to them just get used to it--like the frog in the pot of slowly heating water. When you have a wedding situation--where the culture creates this pressure and expectation that can warp even normal people--the entitled people just ramp it up. To their parents/partner, it may seem like normal behavior for THEM, or at least not outrageous. But when outsiders, who are invited to weddings and tend to hear about the details in advance, hear the litany of demands, they're rightly horrified.
My mom's hairdresser had 3 daughters who caused her no end of drama, from high school sports to weddings. The woman constantly complained about it, but then went ahead and did stuff that either rewarded the entitlement or further fueled the drama, rather than responding sensibly by, say, backing off. I mentioned one situation in another thread--I can't remember the details now, but it was something like, the bride decided on the venue and the mom made a reservation and put in a non-refundable down payment, and then the bride changed her mind and the mom had to cancel and lose her money. And this happened multiple times. After the first cancellation I would've said, "No pressure, have the wedding you want, but I'm not putting any more money down. You pay upfront and I'll reimburse you after the wedding." (assuming the mom really wanted to contribute) At a certain point, the parents/others close to the entitled person are really just enabling them or egging them on, even if they don't consciously realize it. I don't think you can be entitled in a vacuum.