Oh my goodness..As some movie character said, this is "Oprah deep"..:-)

But, jokes aside, the questions you are asking are from such a high spiritual realm that any attempt to start answering them seriously would open the rabbit hole..I do understand the need to ask them though. I ask myself those questions all the time and, as a result, I never stop digging trough all kinds of spiritual teachings and trying to understand the basics of our human existence such as who are we and why we are how we are.

On the question of pain, I don't think pain can be "understood" intellectually. Just like you can't "understand" the flavor of, let's say, a strawberry from its description. I could describe to you in lengths all components and subtleties of the strawberry flavor, but no matter how detailed my description may be you would never "get" it unless you eat a strawberry.

To truly understand what strawberry is you have to eat it. You won't understand it through talking about it. Likewise, to understand what pain is you have to feel it. You can't understand it through your intellect. Intellect is too small to grasp deep and intense sensory-emotional experiences like pain.

Intellectual reasoning and conceptualizing and sensory emotional experiences originate in different parts of the brain that speak different languages. Intellect works in a linear fashion. It operates in the world of form where everything "should" be measurable and where all the dots can and "should" be connected through straight lines.

Emotions, feelings and senses are non-linear. They have their own "logic" that doesn't work in a straightforward way like the linear logic of the intellect.

Therefore, trying to "understand" emotions, feelings and senses through the logic of the intellect is futile. It's like trying to understand, say, the Chinese culture while being attached to the concepts of ethics, morality, beauty, justice, fairness etc of the Western culture or, by using the "logic" of the Western culture and vice verse.

The bottom line, if you want to understand what pain is and why it occurs you need to feel it. There is no other way IMO.

As far as the ego is concerned, as earthly creatures, we need egos, we can't live without them. Ego is simply our sense of our individual identity, a sense of who we are in this world. We cannot belong to earth and not to have some kind of ego. Ego is kind of a mental image of who we think/feel we are. As earthly creatures, we are connected to the physical world of form and our mental image of who we are a.k.a ego is also connected to our physical form, our physical environment, the people and the objects in our life who are also presented in a physical form, to the events that take place in real time and space.

So, ego, in and of itself, is not a problem. It's a necessary part of our life. Our strong attachment to the ego is what creates many problems. By being strongly attached to it or over identified with it, we forget that we are not only earthy but also spiritual creatures. This forgetting creates the illusion of separation when we feel separate from everyone else and also creates a fear of death that gives birth to all other kinds of fear.