I think the cause of suffering is resistance. I see a lot of people here have been saying desire. Perhaps desire is the same as resistance, just by another name. All I know is whenever you resist a thing, you achieve pain, or present pain is amplified. Resisting anything as it is causes suffering; perhaps the key is to adapt. That seems to be where the learning comes in.
When I went thru the childbirth process, what I experienced what that each time I had a contraction, it hurt like a muthafukka, but it always hurt worse when I resisted it instead of just giving in to it. The desire to push against the contraction was to make me feel like I had some control, I guess. To totally acknowledge and accept we do not have as much control as we think we do is quite frightening.
OTOH, pain and growth can be the consequence of resistance. A person who works out can vouch for this. Just think resistance training - a simple example but no less true. The body grows thru the process of making the body push against something. I'm guessing many of us could agree uncomfortable life experiences do the same for the spirit/mind.
I think suffering is unique to us, though. Ever observe animals just being animals? They are always in a present state, literally moving from moment to moment until they die, never having to deal with regret or fear (outside of trying to survive an attack). The ability to desire, I doubt, exists for them because they can't project their thoughts in the future and think about what they want that they do not possess in the present moment. Only we (humans) can do that. They are not concerned about happiness, sadness, suffering, or the like. We are, likely, because we are programmed to seek happiness instead of just appreciating life for the way it is.
I think the children's song Row, Row, Row Your Boat speaks this very subject:
Row, row, row your boat
Gently down the stream (Avoid resistance)
Merrily, merrily, merrily, merrily
Life is but a dream