Greed!
If there were only 2 men left on the planet, they would still fight over what the other one has! Maybe it's nature's way of controlling the population, although I doubt it could have predicted how fast we would develope ways of destroying ourselves!

Lack of critical thinking. Bad science, bad business, bad relationships don't last in the face of critical thinking.....or have a much smaller chance of damaging you, your finances, the ecology.....

Over population. The answer...The emancipation of women. Take them off the cycle of breeding, Educate and empower them to empower themselves and us. I could go on but it's 6.42 am and I need some sleep before a big day.And get a photo up of me so I can go up a level. Hope you enjoy your day

Humans

Money? ie: the Tories and the Republicans

Think the root of all the problems in the world would be the the thought of there being a root. I shall explain. Now talking of problems I assume it means problems created by humans, not problems like earthquakes, gamma rays, black holes, that are caused by natural phenomena and there roots are basically the laws of nature or physics, with the constants. Root is basically essentialism, or reductionism, one hand that there has to be a central cause or root, an essence that causes the problems. Christian religion the root would be sin, so you need a sacrifice to clean the sin, that is the eschatology and cristology, and all the religious coming to your door and spreading the good news, other religions and even philosophies have their own narratives as to roots. In Buddhism the root is desire or rather the grasping or aversion of desire, that leads to karma and this leads to suffering and reincarnation. Political philosophies also try to narrate that humans problems require political solutions, so we got the neoliberals or libertarians that the problem is a big state (forget that most of the history of humanity there were no states therefore no progress) and you need small states, lower taxes and let big corporations take care of everything, the communists is that capital is the problem, or big corporations, again the solution is political, close all big corporations and make the state in charge of everything. The problem with trying to find a root it leads to essentialism, that all objects have an essence so it difficult to leave room for a dynamic, progressive worldview, and the other is reductionism, reducing all problems to one root, so it all requires one solution, which has lead to dangerous religions and political philosophies. I prefer that rather than simple cause and effect, roots, essence, reduction to one factor, explain the world through interdependence, dynamics, complexity, emergence.

Evolutionary mismatch.

[en.wikipedia.org]

Active ignorance masked at morality wrapped in selfishness...

Religion.

Honestly the root of all conflict stems from culture and scarcity of resources.

Failure to communicate

Well, not sure about 'all' of them: But I think the egocentrity of religion is a major issue.

I think there are several, but become more and more prevalent are greed and tribalism