What is the root cause of suffering?

Is it not simply the divide between how we think it "ought to be" and "how it is right here right now" that creates our suffering? Expectations! That is the problem.

How I think it OUGHT to be

How we think things "OUGHT to be" comes from our past conditioning. Nothing wrong in itself with "OUGHT" (sometimes we use the word ego here) ... We need to have beliefs about how things ought to be to know which end of the fork to use when we eat. The problem is that we "camp out there" ... when we believe it is Reality - It it rarely is!

REALITY

Reality is only ever found "right here and right now" - what is so this very moment. The entry into Reality and Truth is always accompanied by feelings of great peace, gratitude, and awe.

Of course, this begs the question: "what is Truth?" When the Truth is realized, it always brings a great shock. The shocking Truth:

To God belongs everything in heaven and earth,
God is in control of all things,
and everything is temporary.

SUFFERING

When the GAP between "how I think it OUGHT to be" and how it really IS becomes large, we experience stress and suffering.

There are two ways to remove suffering ... we can change our beliefs (and accept what is happening right here right now), or we can change our circumstances. In general it is easier to change beliefs than it is to change circumstances.

The best way to eliminate suffering completely is to notice that which arises in the mind is not real!
You will probably have to take up a meditation practice to arrive at this truth.