Don't you think it's nice of the owner to place such a kindly-worded sign on their garage door? Signs can often be unnecessarily rude, seeming to accuse the reader of a heinous offence without them even considering the act.
I recently saw a sign pasted on an office printer: "DO NOT PRINT A4 WITH THIS PRINTER!!!". Someone had clearly tried printing in A4 on our communal A3 printer - and I guess that was a big no-no. However, a more appropriate sign may have read "Please don't print in A4 to this printer, it causes bla bla bla.".
Perhaps it sounds strange, but I felt accused each time I walked past the sign. And, besides for that, the piece of A4 paper taped to the front of the printer simply lacked taste and class (in my opinion, of course). Naturally, I removed the sign in an attempt to restore some dignity to the office. ;)
Next time you create a sign, try making it a little more friendly - or if you're able, include a little humour. The world will be a far friendlier place.