Newton postulated Universal Gravitation as a force acting between bodies. The why and how of it was left unsaid, and Newton remarked, "And to us it is enough that gravity does really exist ..."
However, "it really exists" only as a necessary postulate in "flat space-time", which Newton implicity assumed by his Three Laws of Motion.
Einstein realized that gravity does not act as a force, and orbital motion, e.g. , is explained in the same way as Newton's straight line motion : As the natural motion in the ABSENCE of forces. This is the basis of Einstein's, Principle of Equivalence, which says that gravity is a "fictitious force", just as centrifugal force.
Well, then comes QM, and the decision that gravity "must" be quantized. I can understand the gist of the idea, but I have to recall that Quantum Field Theory functioned very happily in the context of Special Relativity, where Dirac, e.g., inferred the existence of Antimatter, and all the other advances of particle physics took place, and necessarily so since gravity is undetectable at the quantum level.
So I more or less stopped worrying my old gray head about it.
dr_lew I appreciate your reply, but it just reminds me that I dropped out of HS because I COULD NOT understand trigonometry!