Gravity, a weak force? Why is gravity such a weak force? 
It becomes strong for particles only at the Planck scale, around $10^{19}$ $\text{GeV}$, much above the electroweak scale ($100$ $\text{GeV}$, the energy scale dominating physics at low energies). 
Why are these scales so different from each other? What prevents quantities at the electroweak scale, such as the Higgs boson mass, from getting quantum corrections on the order of the Planck scale? 
Is the solution super symmetry, extra dimensions, or just anthropic fine-tuning?
Can we relate few problems of quantum mechanics with gravity ?
Despite the fact that there is no experimental evidence that conflicts with the predictions of general relativity, physicists have found compelling reasons to suspect that general relativity may be only a good approximation to a more fundamental theory of gravity. The central issue is reconciling general relativity with the demands of quantum mechanics. Well tested by experiment, quantum mechanics is the theory that describes the microscopic behavior of particles. In the quantum world, particles are also waves, the results of measurements are probabilistic in nature, and an uncertainty principle forbids knowing certain pairs of measurable quantities, such as position and momentum, to arbitrary precision. The Standard Model is the unified picture of the strong, weak, and electromagnetic forces within the framework of quantum mechanics. Nonetheless, theoretical physicists have found it to be extremely difficult to construct a theory of quantum gravity that incorporates both general relativity and quantum mechanics.
At the atomic scale, gravity is some $40$ orders of magnitude weaker than the other forces in nature. In both general relativity and Newtonian gravity, the strength of gravity grows at shorter and shorter distances, while quantum effects prevent the other forces from similarly increasing in strength. At a distance of approximately $10^{-35}$ $\text{m}$, called the Planck length, gravity becomes as strong as the other forces. At the Planck length, gravity is so strong and spacetime is so highly distorted that our common notions of space and time lose meaning. Quantum fluctuations at this length scale produce energies so large that microscopic black holes would pop into and out of existence. A theory of quantum gravity is needed to provide a description of nature at the Planck length. Yet, attempts by researchers to construct such a theory, analogous to the Standard Model of particle physics, have lead to serious inconsistencies.
 A: I will turn my comment to an answer:
To start with, the first answer is that : this is what we have observed and deduced from experimental measurements.
The particle physics data has been fitted well by the Standard Model, it is practically a shorthand for all observations up to now and very few discrepancies exist with measured values. This model expects unification of all three forces at very high energies, and nothing to contradict this hypothesis has been found. The prominent cosmological model of the Big Bang uses this expectation in translating astrophysical data into the Big Bang  model.
What is missing is unification of all  four forces, gravity included. This is usually called a Theory of Everything, TOE, and is the holy grail of theoretical physics.
A theory of everything in principle will have quantized gravity and string theories do have quantization of gravity  but it still  has too many problems with vacua etc, a working model has not been found . If it succeeds to model existing data,  it will also answer this question, which is called the hierarchy problem. 
So your question cannot be answered within the existing theoretical framework. Once a TOE is found the answer to the hierarchy problem  will be within the model parameters, and then one will be asking "why this TOE" . 
