It is quite unreasonable to assume that as a character gains levels of ability in his or her class that a corresponding gain in actual ability to sustain physical damage task place. It is preposterous to state such an assumption, for if we are to assume that a man is killed by a sword thrust which does 4 hit points of damage, we must similarly assume that a hero could, on the average, withstand five such thrusts before being slain! Why then the increase in hit points? Because these reflect both the actual physical ability of the character to withstand damage - as indicated by constitution bonus - and a commensurate increase in such areas as skill in combat and similar life-or-death situations, the "sixth sense" which warns the indivdual of some otherwise unforseen events, sheer luck, and the fantastic provisions of magical protections and/or divine protectionů.
Consider a character who is a 10th level fighter with an 18 constitution. This character would have an average of 5.5 hit points per die, plus a constitution bonus of 4 hit points per level, or 95 hit points! Each hit scored upon the character does only a small amount of actual physical harm - the sword thrust that would have run a 1st level fighter through the heart merely grazes the character due to the fighter's exeptional skill, luck, and sixth sense ability which caused movement to avoid the attack at just the right moment. However, having sustained 40 or 50 hit points of damage, our lordly fighter will be covered with a number of nicks, scratches, cuts and bruises. It will require a long period of rest and recuperation to regain the physical and metaphysical peak of 95 hit points.