The Natural Mind
While I do not always agree with much of what Andrew Weil believes in (especially lately), I found his book The Natural Mind had some very interesting perspectives on the role of the altered experience. It has been many, many years since I read this book; it was out of print for a long time but has recently been re-issued.
Central to his thesis is that it is a natural drive of our mind to experience different perspectives for very much the reasons you cite.
Whatever Mr. Weil's experiences or medical beliefs, he looks pretty healthy and happy when I see him on the tube.
(As for my own altered experiences...well, all I will say is the best ones have been through meditation or dreaming.)