It's basic survival instinct for humans to forget and move on.
But how quickly we forget.
As 2015 begins to slip into history, it's plain to see that this was a violent year - a year where misguided wrath was given a chance to really bloom and cause an infinite amount of damage.
- It all started at the end of 2014 with the massacre of 141 innocents, among them 132 schoolchildren, in Peshawar, Pakistan.
- Then, the violence moved on to Paris in the first week of 2015, when two Islamist gunmen walked into the offices of the Charlie Hebdo magazine on 7 January and killed a dozen people, including two police officers. The satirical magazine was targetted because it had published irreverent cartoons of the Prophet Muhammad.
- Then, in March, more than 140 people were killed in suicide bomb attacks on two mosques in the Yemeni capital, Sana'a.
- The next outrageous attack came in a suburb of Tunis on the morning of 18 March, when three terrorists attacked a party of European tourists visiting the Bardo National Museum. The shooting was indiscriminate, with two Tunisians among the 21 killed. Ironically, at the time of the attack, the Tunisian parliament was debating anti-terrorist policy.
- In April, there was an attack in Kenya, which left 148 students dead.
- Three months later, a lone terrorist equipped with a Kalashnikov assault rifle opened fire on tourists at the beach resort of Port El Kantaoui, about 10 kilometres north of the city of Sousse in the same country. As a result of the shooting, 38 people were killed, 30 of them British.
- All this was a grim precursor to an even bigger and more lethal attack in Paris. On 13 November, 130 people were killed in a series of terrorist attacks, including 89 attending a rock concert.
In most of these cases, the Islamic State has been at the forefront. It began the year by beheading Japanese hostages, burning a Jordanian pilot alive in a cage and announcing the death of American captive Kayla Mueller.
By spring, the terror group was really on an evil roll, taking over the Iraqi city of Ramadi and the ancient town of Palmyra.
The group then claimed responsibility for downing a Russian jetliner in October, drawing the Russians into the bombing campaign.
Just a few weeks before that, 102 people were killed in Ankara when a peace rally was disrupted by a double suicide bombing.
It has also been a year marked by mass shootings, in places like Charleston, South Carolina; Chattanooga, Tennessee; Roseburg, Oregon; Colorado Springs, Colorado and San Bernardino, California.
The last happened two days into December, when a young couple - Tashfeen Malik and Syed Farook - killed 14 people.
In all, 12,700 people have been shot dead in the US since the beginning of this year.
Guns have killed four times as many people in ONE country as jihadi terrorism has in the whole wide world.
According to a recent study, "mass-fatality terrorist events" - in which more than 100 people are killed by terrorists in one country in one day - occurred just over four times per year on average between 1978 and 2013.
In 2014, there were 26.
But in 2015, the number has been much higher than 26 - depending on how many of the massacres carried out by ISIS and Boko Haram, the extremist movement based in Nigeria, are counted.
According to a Wikipedia list of Islamist terrorist attacks in 2015, there have been 105 attacks, with a total death toll of around 2,800. That list looks very incomplete. Syria, for one, has been completely excluded from it.
Violence in 2015 also found another dimension: a new geographic range.
In November alone, there were the Paris attacks, bombings in Beirut which killed 43, plus the downing of a Russian passenger jet over the Sinai with 224 people on board, a raid on a hotel in Mali, attacks in Nigeria killing more than 80, a dozen or so in Somalia, many more in Iraq and Syria, significant violence in Afghanistan and stabbings and shootings in Bangladesh.
It's a chilling fact that in the year that stretched from Charlie Hebdo to the Bataclan in Paris, it's Nigeria that has had to confront the biggest increase of terrorist casualties ever experienced by a single nation.
And that's all thanks to Boko Haram, the Islamist group which has links with the ISIS. Let us not forget that during the course of a six-year reign of terror, more than 20,000 people have been killed in Nigeria and a further 1.23 million have been displaced.
Even journalists covering such events were not spared. "Of 69 journalists killed for their work in 2015, 40% died at the hands of Islamic militant groups such as Al-Qaeda and Islamic State," reads a report released this week by the Committee to Protect Journalists.
Given the way things are going, there's no doubt we can expect much of the same, if not worse, in 2016. At least until threats like ISIS and Boko Haram are tackled.
But even if they are, it is the nature of the world to throw up more crazies willing to unleash wrath upon the human race.
That's not pessimism. That, unfortunately, is the truth.