Why do they do it?

Im not in a good mood today. I got into the office this morning to find my chair hidden in a cupboard, one of my keyboards plugged into the mouse port of the computer, and my desk re-arranged with my own personal bits and pieces moved around and mucked about with. Oh hilarity. Oh comic genius. Why dont people leave my stuff the f*ck alone? Its not the first time this has happend - and this time I emailed the dept. to ask them who did it and to tell them I do not find it the least bit amusing. Now alot of you might think "Oh dont be so miserable it was just a joke" but no. I do NOT find it funny. I do not touch other peoples personal possessions or computer equipment when they are not in the office, so I expect the same courtesy to be extended to me. People probably read the email I sent and thought "miserable sod" but I really could not give a toss what people think about me. Im here to do my job to the best of my ability, be professional and courteous around the office, then go home. Im NOT here be laughy smiley happy bubbly person around the office. Sometimes I think "give me a tall tower and a high powered rifle and nothing to live for..."