Can anyone please explain the basic most fundamental reasons behind fundamental forces, i.e. what causes electromagnetic, nuclear and gravitational forces.
Physics is not mathematics. It is an observational discipline that uses mathematical formulations to fit observations and predict the behavior of new set ups.
Experimental observations can be fitted very accurately by defining four fundamental forces and the way elementary particles interact under these forces, weak, electromagnetic, strong and gravitational.
They are fundamental because they are the foundation stones of the standard model theory, which is the underlying theory of all observations we make in our experiments. Quantization of the gravitational force is still under research for the particle model, but works extremely accurately classically.
You are essentially asking "why" four fundamental forces.
The answer is "because this is what we have observed".
Physics does not answer "why" questions , only how from certain assumptions and using mathematical formulations one can describe physical systems. Part of these assumptions are the four fundamental forces. They are among the simplest assumptions to make in fitting experimental/observational data with mathematical models, which models allow us to have predictive power over the behavior of matter.