US Tourist jumps off the top of Chain Bridge, why??

My gut answer is it's because he was an idiot.  However, it's probably more akin to the fact that he was of the opinion that he can do anything he wanted, when he wanted, without any thought of the potential impact not only on his own family and friends but also on the people who have to clear up the mess afterwards.

I think some people also feel they can get away with anything on holiday, like normal rules of law do not apply to them.
Maybe he grew up watching too many video games and thinks reality is a game.
An old school mates older brother was being silly in his early 20's at the Kern River in Ca.
This happened ages ago, maybe nearly 40 years back now or more.
The park area at the time did not have any warning signs up and that one week alone there had been a couple of deaths and a few serious injuries but still no warning sings up at one spot that tempted crazy adventure seekers to dive into the river.
Her brother drove into the river and hit his head on a rock and broke his neck.
He became a quad and couldn't even use his arms, a bit of movement only in his fingers.
He became very depressed and finally talked one of his friends into giving him a hot shot of heroin to end it all, he just couldn't go on living in a wheelchair and not being able to do anything for himself.
Ironic now that this old school mate also finds herself in a wheelchair, due to some nerve damage in her legs or something, not even sure, they doctors cut her so many times and gave her a new knee, fixed up so well that now she can't even walk on her own without using a walker in the house and wheelchair in public.
Off topic with her sad tale, however I suppose this jumper got off easy, would be worst to end up a quad or having to use a breathing tube for years and years.
Natural selection I suppose...

I actually don't think human nature has changed much. Some of my childhood peer group broke bones doing jumps on their bike trying to emulate Evel Knievel who was popular at the time.

The only real change is today's copy cats, with (anti-)social media, simply have more options to select from.

In the end the results are potentially the same: candidates for a Darwin Award.

Darwin Award! Yes, like those fools who eat washing pods.
Mom did make us eat soap a few times as kids for speaking like a sailor  but just a good short wash out, didn't have to eat the bar
( It really didn't work too well though, I still can have a foul mouth when needed.)
Evel Knievel more then likely did his homework before attempting anything too insane.
Somehow I know I should feel sorry that someone killed themselves by being wild and free but really it wasn't wild and free, it was just dumb.

Were there drugs found in his system?

I forget the term, but its when people jump off cliffs. They are told time again thats its dangerous and yet they still do it.

Have to feel a bit sorry for this guy's family though, to know you wasted all those years raising someone who couldn't care less about their life must really sting.