X can really stand for anything, such as wx in weather, mx in maintainence, or x in crossing. It's not used much, and when they picked it they probabally just used it to fill space. When you think about it, it wouldn't really sound any better if other letters were used.
As Ywg_jy correctly pointed out, Canadian airports codes are 4 letters, each beginning with the letter C. This dates to the time when the Canadian government was allocating radio callsigns. When they allocated callsigns to those in the form CYxx and CXxx to airports. (A radio callsign is just an identifier, like WNBC or KTLA) The 3 letter airport code for a Canadian airport is just the callsign minus the 'C' at the beginning. So Toronto's airport radio callsign is CYYZ and the 3 letter airport code is YYZ, just like Vancouver's is CYVR and YVR. It was just an adminstrative decision. Other countries used a different method to pick airport codes, this was Canada's.
The motto for Ft. Myers (SW Florida Reg) is "Ready to Serve the World. They have it written all over their website, and I think they have that code for that reason. Or maybe LoneStarMike is right, and they just figured out that their assigned IATA code worked out as an abbreviation for a cool moniker.
Not all Canada airports start Y, only the main ones. There is a small 'airport' on the area around 10 miles south-east of YVR known as the Delta, called Boundary Bay, code ZBB. Montair are based there, a small regional carrier with a fleet of one Pilatus PC-12. It's an interesting field to visit at weekends (when YVR gets a bit quiet) as there are a few veterans (HU-16 etc) amongst the 150-plus residents.
Re LGW, LHR and STN - Heathrow and Gatwick have been the designated London Airports for years now, since the 40's and 50's I believe - Stansted was a war time military base and turned into the official 3rd London airport back in the 80's - I would assume that is why the coding is different.
Re LAX - short for LAXative - just passing through!!!