What's your reaction when you hear about yet another mass shooting?

I am saddened and angered. I think I will go buy an arsenal to protect my home and get a concealed carry license.

Just another day in America...

Sadness, mostly. I think about the loved ones of all involved. 

Resignation. Congress is willing to sell out to the gun lobby and responsible gun owners, who, IMO, are the ones who should be demanding change, do nothing. In this country, so many people think they that if they want something, they should be allowed to own it, no matter what kind of effect it might have on public health and safety.

I thank God I live in a nation with sensible gun laws.

My 1st question is why? 

In this case the shooter had identified psychological problems.  How he was able to purchase a gun baffles me.  If existing laws had been enforced, this may never have happened.

We are now going to see a whole boatload of non-efffective legislation put forward to no effect,

Enforce what is on the books and let's go from there

Angered and saddened but also baffled why it happens.

Parts of Mexico and Brazil are extremely dangerous, mainly by drug gangs but also the police in Brazil, if you are a homeless child.

However, sticking to stable countries and excluding civil-war, gang-wars and terrorist acts; random attacks with no obvious motives are extremely rare outside of the USA. There, they seem so common a Utah-resident correspondent elsewhere tells me that many are not reported widely even within the country.

The one or two (I am not sure) in New Zealand a few years ago were by a far-Right extremist on  Muslim targets, so could be classed as terrorism.

Two in Norway, in the last 20 years: a car-bomb in Oslo and separately, the attack by a political extremist gun-man on a youth holiday camp run by a political party of opposite but mainstream, not extreme, beliefs. The latter at least could be considered as "domestic" terrorism as the killer was a politically-motivated Norwegian in his own country - though still a cold-blooded, pointless murderer.

Of non-political incidents, I think the UK has suffered 4 in the last 40+ years. The worst was the attack on Dunblane primary-school 25 years ago, where 13 children and a teacher were shot. The others were street incidents.