Why do the space time get curved around a massive object?What problems do we face if we do not consider the space time to be curved? As far as I have the knowledge of GTR that a mass bends the space time around it.But why does this bend occur?The example from real life that when a mass is placed on a net then the net bends but it us very difficult for me to visualise the situation of bending of spacetime due to a mass.What is actually happening?What is the physical basis behind this bend?I want to say that the books which I have studied till now just say that the space time "just bends" around an object and even my teacher told me that it just bends without any explanation so I just want to know the reason for the occurrence of this bend.What is the need for spacetime to bend around an object?What physical problem we can encounter if we do not the bending of space time.
 A: As photons have energy, gravity affects light rays, turning their path from straight to curved, and changing their energy (frequency/color).
In classical relativity light always travels in a straight zero-lenght line, with phase speed = $c$.
If you include gravity in your relativistic model you this is not true anymore. As this is an important property of the model that helps verifying that the theory is sound, instead of using a model where you cannot trust light going straight, you change your definition of "straight" so that it stays coherent with the fact that light moves in a straight line. That's where the "curved spacetime" comes from.
So the logical process is not:


*

*I observe some strange physical phenomenon from which I conclude that the spacetime is curved

*I know that light goes straight, but the space is curved

*the light curves


but instead:


*

*I observe light going around big masses

*But I know that light goes in straight lines

*I either have to change the idea that light goes in straight lines, or that the space is plain

*I prefer keeping in my model the property that light goes in straight lines, and from my biased point of view the space is curved 


This is consistent to the idea beyond special relativity, in which Einstein decided that instead of preserving the rule that speed adds up while changing the system of reference preferred keeping the rule that the speed of light is the same in all (inertial) system of reference.
A: In general relativity the fundamental equation is roughly $$\textrm{Curvature} = \textrm{Matter content}$$ .In the context of general relativity it does not make sense to ask "why" matter curves spacetime, because this is the most basic assumption in the theory.  
It makes more sense to ask why one would try to construct a theory where gravitation is a geometric theory. The answer to that question is the equivalence of gravitational and inertial mass: the dynamics of a particle in gravitational fields are independent of the particle mass.
Accepting that the equation for gravitation should be something like $$\textrm{Geometry} = \textrm{Matter}$$ one can make physical arguments for why he left-hand side should be (a particular measure of) curvature: the curvature has the the right number of derivatives (two), it is zero for the spacetime of special relativity, it has the correct number of components, and using the correct measure of curvature, conservation of energy is automatic. 

