Throughout the life of this series, the development team at Bioware has told us that choice matters. From the first game to the third, your choices as Shepard have had a very real impact on the state of the galaxy and those within it.
From making large decisions such as deciding to destroy or salvage a Collector base all the way down to deciding to treating an annoying reporter with respect or not, the doctrine of this series has always been that your decisions as a person (personified through Shepard) matter in ways that reach far beyond the moment in which they were made.
However, the ending of this trilogy feels a bit like the Bioware dev team is an evil kid toying with a bunch of ants on the sidewalk. That, no matter which way you turn, the end will always be the same. That there is something watching over us all that renders our choices moot. There is no hope. There is no salvation. There is only the illusion of both for you and I and everyone else that we know.
In the end, life serves no purpose beyond the goals we set for ourselves to achieve some sort of imaginary end. Because, in the end, we all achieve the same goal - death.
ME3's ending is such a bummer because of what Shepard represents for all of us. Hope. Perseverance. Sacrifice. In two out of the three possible endings, sacrifice is the answer, but at what cost?
If Shepard's life were my own, the thought of the ones that I love, along with all other organic beings, being stuck wherever they are (as a result of Mass Relay destruction) is not a comforting one. There is no salvation for them - only pain compounded onto pain.
It sure seems that Bioware, as a group, doesn't believe much in happy endings, but why prop this whole story up as if the fight actually matters, when very little does? Be it Reaper destruction, Reaper control, or organic/synthetic consolidation, none of these endings change much. The world that everyone knows and loves has been completely torn apart.
As a person, I want to believe in the power of doing right by others, often at the sacrifice of myself. And I want to be motivated by others doing this. But Bioware did a wonderful job of making me feel like nothing I'll do in my short, insignificant life will ever actually be worth a damn. And that sucks. I guess it doesn't make it not true, though.