Why mass & energy bend spacetime? I understand how light / matter bend spacetime but I'd like to understand WHY. Is there some kind of interaction?
 A: "Why" is really a question for philosophy.  Science describes what we see.  It happens to be that, in many cases, when people ask "why" they are really asking for a lower level scientific model which demonstrates something, but when you get down to Quantum Mechanics and General Relativity, we start to have to admit that these simply are the best models we have, and can't go any lower.  The next step is always philosophy, not science.
That being said, you can think of mass as a property derived from the ability to distort space time.  Thus, asking why mass bends spacetime is like asking why a red object is red.  It's red because that's the property which defines red objects.
It actually turns out that mass in GR is not simple.  From wikipedia:

Generalizing [special relativity's definition of mass] to general relativity, however, is problematic; in fact, it turns out to be impossible to find a general definition for a system's total mass (or energy). The main reason for this is that "gravitational field energy" is not a part of the energy–momentum tensor; instead, what might be identified as the contribution of the gravitational field to a total energy is part of the Einstein tensor on the other side of Einstein's equation (and, as such, a consequence of these equations' non-linearity). While in certain situations it is possible to rewrite the equations so that part of the "gravitational energy" now stands alongside the other source terms in the form of the stress–energy–momentum pseudotensor, this separation is not true for all observers, and there is no general definition for obtaining it.1
How, then, does one define a concept as a system's total mass – which is easily defined in classical mechanics? As it turns out, at least for spacetimes which are asymptotically flat (roughly speaking, which represent some isolated gravitating system in otherwise empty and gravity-free infinite space), the ADM 3+1 split leads to a solution: as in the usual Hamiltonian formalism, the time direction used in that split has an associated energy, which can be integrated up to yield a global quantity known as the ADM mass (or, equivalently, ADM energy).2 Alternatively, there is a possibility to define mass for a spacetime that is stationary, in other words, one that has a time-like Killing vector field (which, as a generating field for time, is canonically conjugate to energy); the result is the so-called Komar mass[3] Although defined in a totally different way, it can be shown to be equivalent to the ADM mass for stationary spacetimes.

If that seemed hard to read, if I may humbly present a takeaway: trying to explain mass in general relativity, much less "why" mass is the way it is, leads one to spend 2 paragraphs of difficult to dissect phrasings to admit that its hard to even define "mass" in GR in the first place!  It is an exercise left to the reader to decide whether it is in their interests to dive any further than that.
Myself, I find the philosophical questions regarding motion to be fascinating, and they're easier to digest than GR, which requires a great deal of formalism to explain anything thoroughly.  For example, I find it very interesting to look at Aristotle's concept of Motion to be a worthy read because it's so different from ours, and yet is seen as the groundwork for the invention of Calculus and the modern physics that followed.
A: As this concept was originated by Einstein,he thought the Newton's law of gravitational is a classical case of two body attraction but for the large scale he generalize the notion of Newton.He thought that space-time is like a trampoline pad and if there is some object(mass)  on a trampoline pad it will definitely bend  the trampoline pad and this bend in space time is known as "curvature of space- time" and the object that fall in that bending region feels attraction towards the other body.Now, its depend on who will attract more towards the other by making the simpe concept that "larger the object mass larger will be the space-time curvature".Therefore, usually small object rotate around the larger one like earth rotate around the sun because sun has large bend on the trampoline pad rather comparable to the earth.
A: Given that you understand the how part, I guess you also realize that the how part is actually "how much". I mean you realize that what you understand, is the quantitative analysis, not the root cause analysis.
And I agree with you that quantitative analysis does not always reflect on the root cause. But if someone tells you the root cause, then you will ask what is the "root of that root" and it will be never ending.
Therefore, while it is good to think about the why part, you have to figure it out yourself. When you do that, then your mind will settle on it, otherwise, you will always have the next "why".
Hint to you is that the answer will be something that will explain its own root along with itself. Meaning the answer will be rooted in itself. Alternatively, you can master the concept mathematically, and you will not bother about the why part. Just like, you never ask why 2+2 is 4?
