ME3 Ending Rant

If you really thought it was good, well good for you I guess.
Not everyone holds the same expectations and standards, but to answer your question we need to review why many people find it bad.

The first reason would be because it was a bit lackluster, there was subjectively no 'solid' ending, but rather just a try-hard quasi-philosophical one.

Then there's the fact that the "multiple" endings aren't as explored as they should, it's only >implied that they are different. The fact that a good chunk of the end cutscenes are almost identical with different colors/effects only worsens it.

You must also consider, should you have finished the game with the extended cut DLC, that the actual original ending had many plot holes, felt unfinished and there was an announcement for DLC at the fucking end.

For me I hate that it feels "tackled on", sure there were some parts of the trilogy where you had to face the whole "organics and AI" subject, but it was mostly contained to side things and the Reapers themselves were always regarded more as space monsters than just AI.

The actual original plan was that the dark energy sub-plot from ME2 (remember Tali's recruitment mission?) would come to play and so the galactic would ironically be digging it's own grave by the thorough use of mass effect, used by biotics and FTL (faster-than-light) travel. It feels much more original and to some extent quite more believable than just "badly programmed AI".

And as a final point, I have issues with the "space magic" in the ending (and majority of the plot as well). See when you make a believable universe you must write plots that makes sense within it's lore, like how we already established that element zero allows biotics to be a thing. But then comes the Crucible, which is completely unexplained how it works and why, it just works in favor of the plot, there's the Statchild/Catalyst which feels really forced and overall very pretentious.

Oh and don't forget how this whole ordeal came to be.
The original ending had leaked online (dark energy plot), so the story goes that EA decided to heck with it, take a small part of the dev team and force them to make a new ending in like 6 months or so. You can guess what happened.

Perhaps you enjoy the ending because you don't get too much into the technobabble revolving lore, space magic, perhaps you don't hold very high standards to notice plot holes here and there (like the Normandy hovering for 5 minutes in front of Harbinger just so you can say goodbye to your companions), you just value what you see and hear first without much thought to it, no offense.

Oh and I forgot to add, the fact that there's pretty much zero epilogue also shits on it, you're only told what supposedly might happen and left on a cliffhanger, while BioWare pretentiously claims it was so that "it's up to the players to imagine what happened afterwards".
Remember that the ending actually renders many important choices from the trilogy meaningless, such as the council from ME1 or the Collector station from ME2.