its been stated plenty of times..most recently when eddie brock hands his symbiote over to that mob boss's kid...that the first time you bond with the alien, it hurts like hell....the alien just doesnt cover the body...it genetically graphs itself to a persons adrenal and nervous system with a painful amount of force....honeslty i think it hurts everytime...but eddie has simply gotten used to it since they have been spereated so many times....as far as seperating from the alien i believe the pain level is up to how willing the symbiote is to leave the body