Why is chess against a human fun and against a computer so boring?
Because with the computer, no matter how hard you try, you will lose. You can't take comfort in beating a machine because they calculate millions of moves per second. Playing with humans is much easier because there is a chance that they will make mistakes or beat them because of time or fatigue. Also if you beat a human it means that you are superior chesswise.
Because when you play against a human you have the illusion that you can win, on the other hand when you play against a powerful chess program, you know in advance that you are going to lose.