The thing you are asking for, the "why does mass cause spacetime to bend" seems... perhaps impossible to answer? Or like, I don't see why there should necessari

The thing you are asking for, the "why does mass cause spacetime to bend" seems... perhaps impossible to answer? Or like, I don't see why there should necessarily be anything that satisfies what seems to be your reason for dissatisfaction. Imagine we explained it as "Mass causes X, and because X causes spacetime to be curved, we have that, mass (indirectly) causes spacetime to be curved." . But then, why would you be any more satisfied with this? Why would you not then ask, "Well, why does X cause spacetime to be curved?" ? (Or, "why does mass cause X?") Now, perhaps there is such an X, and perhaps it will be found. But, I don't see why this would satisfy you any more than "Mass causes spacetime to curve" satisfies you. If you keep asking "why", you either eventually end up in a loop, have an infinite regress, or stop getting an answer. A non-empty directed graph either has a cycle, an infinite outgoing path, or a vertex without an edge away from it. (And also, either a cycle, an infinite incoming path, or a vertex without an edge going to it.) Many are content to allow the laws of physics to just be, without explanation, others may say that the laws of physics are explained by God, who is without explanation. Personally, I go with the latter, but, unless one wants to go with infinite regress or a cyclic explanation, one has to allow that something is without explanation. For your complaint to be compelling, I think you should give criteria for, what conditions would something have to satisfy, in order for it to satisfy you? (Also, you seem to assume that if the two particles suddenly acquired mass, that they would immediately begin to feel a force between them. While the hypothetical is presumably impossible, still, what I imagine happening would instead by that there would be a light-speed delay between when they gained mass and when they began to accelerate towards one-another. Though, I'm not sure if there is a fact of the matter as to "what would happen in this impossible hypothetical".)