I'm confused! Why would anyone want the sad ending?? Lol I am playing to see Yuna bring Tidus back! Otherwise its kind of a boring game and I probably wouldn't play it. I hope I didn't offend anyone. If so, sorry.
So, what do I have to do to get the ending I am yearning for?
Some people think that it's more realistic. I don't like it though, this is fantasy - it's not exactly realistic that he was a dream in the first place.
I'll make this vague so as not to spoil anything. Basically, everytime you hear a whistle in the game press x. Every single time. Including at the end. That will get you the ending you saw on youtube. It's impossible to get the perfect ending on the first playthrough as you can't get 100% which you need to get it, but basically in order to get it you must have done absolutely everything in the game that there is to do, every side quest to the fullest basically. Like I said though, it's impossible the first time. On your second playthrough do everything you didn't do on your first go, if you had to choose between two things choose the other one this time around, and it should reach 100%.