The more you smoke the more it takes you to get high is the norm to me.
40 years ago I guess the stuff you smoked was not as strong as the high thc skunk that you get nowdays.
You may smoke a good few spliffs for many years and have no ill effects.
Some people had smoked 30 cigarettes a day since they were 14 years old and live to be old but many didnt. Smoking in Europe has been banned inside public buildings because of the people who were crippled and died through smoking not the ones that it had little effect on.