In simple words:
We live in the universe we find our selves in, and do experiments with the available materials. These experiments have established , in time of discovery sequence, gravity (Kepler, Newton), electromagnetism( Faraday ...Maxwell), strong interactions (seen as nuclear) and weak interactions ( seen in decays of nuclei as beta decay).
Once differential equations entered the modelling of physics data, and were used as predictive tools the models became complicated . As experiments progressed and predictions of classical models failed to describe the new data, new models were sought. Quantum mechanics was invented to fit the microcosm data ( and not only, think superconductivity).
The continuous target of theoretical models for physics is simplicity, and the beautiful format of Maxwell's equations, which united what eighteenth century physicist thought were two different forces: electric and magnetic. Unification of forces reduces the number of necessary constants and is much stronger in predictions, as for example Maxwell's equations "predicted"/clarified that light was a combination of electric and magnetic fields, not an independent observable.
In a nutshell, this obsession for unification of forces led to models for unifying, from the four forces listed above, the weak with the electromagnetic in one mathematical model using differential equations, because the symmetries observed in particle data gave a strong hint that this could be done.
When the model was proposed it was necessary for the higgs mechanism to be invoked, just for the weak and electromagnetic, to posit that at very large energies the exchanged particles representing the forces, W and Z and photon, were all massless and by a symmetry breaking mechanism acquired the masses we now measure in our laboratory experiments. Below the symmetry breaking scale the elementary particles in the table acquire masses (specific and constant).
The masses of composites of these, protons, neutrons, ... and the tables and chairs, are masses resulting from the four momentum vector additions of the constituents. The strong interaction exchange particle in the current standard model always has a zero mass ( gluon) as it is never free in our energy regime. The proton bag
It includes innumerable gluon exchanges and quark antiquark creations. These obey the quantum chromodynamic force , and there are calculations which give close to the mass of observed hadrons, in a theory called lattice QCD.
The unification dream is strong in the theoretical researches, and it is proposed that at very high energy all three, weak, electromagnetic and strong are united in one force , as implied by the data from coupling constants of the three interactions :
Gravity, with which you start, is at the end of particle considerations. It comes much later because it is such a weak force compared with the other three. The unification of all four forces is the holy grail of researchers in particle theoretical physics, called Theory of Everything. TOE.
First of all gravity must be rigorously quantized, and then a mathematical model found which will embed the standard model, higgs and all, and include gravity, and describe and predict observations that depend on it.
In my opinion string theory theories are on the correct path, since they already have a niche for gravitons and gravitational interactions and can embed the standard model.