Runner's high definately exists.
Its not just one single type, but
several different kinds.
I'd include the warm afterglow after
a nice run, the second wind, feeling
like year can run effortlessly,
intensification of the senses,
senerity of being in touch with
the outdoors, and others.
I have no opinion whether this has
anything to do with brain chemistry.
I also thought its was more due to
pushing the body at a higher rate.
These feelings were more pronounced
the first couple years of running in my
experience. After several decades running
is still a pleasure and daily urge,
but not an infatuation. I tell newbies if
they can get past the first few weeks of
aches and pains, they'll discover the pleasure.
This is true of many sports.