Let me tell you a little story:
Once upon a time, a man decided to climb a mountain. He took a nasty fall, badly bruising himself, and landed in the woods next to a very shaggy dog. Despite his injuries, he limped back to his house, where he left the dog, then to the nearest hospital, where he got some x-rays. When he got home, the dog looked hungry, so he made a steak just for the dog, and turned on the television. He was just about to call the pound when he heard that a wealthy couple, on vacation in the vicinity, had lost a very shaggy dog, and were offering a very large sum for his return. He bought a plane ticket, but fell short on funds. Being a thrifty man, never wanting to live in debt, he sold a chair from his house to pay for the ticket. When he got on the plane, he found that he couldn't take the dog without preparations; the airline, however, was willing to transfer his ticket for a nominal fee. He was forced to pay this fee, and the veterinarian's bills, with a credit card, which irked him even though he knew the reward would offset it. Then he flew to the city in question, but since he was only twenty-four, had to walk ten miles through the woods, going in the general direction of the manor. When he arrived, he found he had missed the front gate entirely. He walked directly up to the door with the dog and rang the bell... when he and the dog were shot dead by a guard.
That's basically what happened here. There were hundreds of character you met, got to know, maybe fell in love with. There were dozens of worlds, detailed, populated, living places. There were thousands of decisions you carefully considered, then acted as best you could with the knowledge that it would matter, that everything you did influenced the universe around you. Then you died, the mass relays broke, galactic civilization collapsed, and none of it mattered anymore. Everyone on the Citadel is dead, so none of what you did for them matters. Every Quarian who isn't already on Rannoch is dead even if you made peace with the Geth or destroyed them utterly, because the migrant fleet joined the fight around earth, where there's not so much as a dextro food supply to be had. The Krogan, no matter who you chose to lead them or whether or not you cured the genophage, are doomed because all the females are on Tuchanka, and the leader was on earth; best case scenario the new trapped horde bombs itself back into nuclear winter while the soldiers and mercenaries everywhere else die off slowly.
And in the last scene, with the little boy and the "stargazer," it's revealed that humans have forgotten how to go to the stars at all, and alien races have faded into legend. So no matter what you do, the best case scenario is a new dark age.
The Mass Effect series has always been about people, choices, one man shaping the galaxy. The ending throws all that away, declares everything you've ever done moot, and tacks on some "art" so the writer could pretend he was better than you.
