The combination of overwhelming intelligence, paranoia, and megomania all in once place. Like gas onto a fire.
My usual prefered line is :
Mark 'Rent-Boy' Renton's (Ewan McGregor) "choose life" diatribe: ("Choose life. Choose a job. Choose a career. Choose a family, Choose a f--king big television. Choose washing machines, cars, compact disc players, and electrical tin openers. Choose good health, low cholesterol and dental insurance. Choose fixed-interest mortgage repayments. Choose a starter home. Choose your friends. Choose a three piece suit on hire purchased in a range of f--king fabrics. Choose DIY and wondering who the f--k you are on a Sunday morning. Choose sitting on that couch watching mind-numbing, spirit-crushing game shows, stuffing f--king junk food into your mouth. Choose rotting away at the end of it all, pishing your last in a miserable home, nothing more than an embarassment to the selfish, f--ked-up brats you have spawned to replace yourself. Choose a future. Choose life...But why would I want to do a thing like that? I chose not to choose life. I chose somethin' else. And the reasons? There are no reasons. Who needs reasons when you've got heroin?")
More great speeches:
http://www.filmsite.org/bestspeeches14.html