1988 honda accord 5speed trans

Driving home I suddenly lost 5th gear. No noise or warning it just disappeared. This has happened before and i found the main shaft end nut to be loose. I retourqued it to specs and it ran fine. A little whine in 5th buy nothing major. Im about to see if I can fix it the same way but my question is what causes this to happen? Last time the syncros looked fine and couldnt find anything abnormal except the end nut were loose. Ive looked at warrantied used transmissions but dont know if going that route would put me any better off. Any ideas? Sure would appreciate any guidance. Thanks. By the way the engine runs flawlessly