Inner resistance.

Some philosophers and spritual teachers say that this is the root of all suffering. Some even go so far as to say it's the esoteric cause of physical ailment as well. I'm not going to get into that. I'm only going to speak from a personal, anecdotal, psychological point of view.

Regardless of whether you believe in the objective existence of the world around you, there is no denying that YOUR experience of reality is entirely and by definition a subjective one. Your perception of what you see, hear, smell, taste, and feel... those perceptions are shaped over time by your previous experience and neuro-conditioning. Spend a few hours at a landfill and you won't even smell it anymore. Spend some time working under a noisy fan or machine, and before long its repetitive sounds fade into non-existence. Our brains take in sensory input and process it into experience, and consequently thoughts and feelings, which are all shaped by our previous conditioning.

Is this why we humans have such difficulty being open to ideas about reality that dissonate with our own sensory input? Maybe. But just maybe, how things SEEM to be aren't really how they actually ARE. The universe is hard at work all around us, flowing and swirling like air currents on both a cosmic and an infinitesimal scale, in all it's marvelous complexity. Change is the only constant. With all this uncontrollable and unfathomable flux, why do we tend to hold so tightly to feelings and ideas of how things should be?

"That asshole cut me off. He should never have done that."

"Why haven't I found love yet? I should be married by now."

Something deep inside our egocentric brains generates these negative thoughts and feelings, this inner resistance to the state of reality at this moment. I mean, every particle in the universe IS already where it IS. All energy potentials are already in their current state. You'd be insane to believe otherwise. No amount of negative emotion or action or self pity can undo what is already done. You don't have to feel resignation, but don't look backward wishing things had been different. Take a breath. Accept that what already is, is. Find peace with the current moment. Then you can move forward.