Why is it so hard to kick start my WR250F - 2003 Yamaha WR 250 F

Is the bike on the side stand when checking ? If it is most probably that is the reason why, try keeping it level.

it depends on how you ride the bike.. If you are constantly ridding it wide open you should check them more often.. but the usual time to check them would be around every ten or so hours.. if the bike dont want to start for you the valves are probally tight so check them

if this is a two stroke,then if you have run it out of fuel,there is no lubrication for the piston and you may have damaged the piston or piston rings,check the compression,esay test is remove sprk plug pit your finger hard into the hole and crank the bike over,if your finger gets pushed off with loud crack sound then compression should be ok,but if it doesnt and the preesure is very weak then you will be looking at a top end rebuild,hope this helps

it will fit

I recomend you to take a compression test.
From your explanation it seems there's no compression at all thts why the engine rotates easier on kick start.

maybe the carburator is dirty? fuel filter clogged?