Is there a scientific reason for me having a high tolerance to many drugs, and alcohol?

YEP Genetics. It is well proven genetically that some people have a higher tolerance to drugs and their effects than others do. Thios is why the practice of medicine is as much art as it is science.
Biological variance. Everybody has some variation on how various substances effect them. Some drugs, including alcohol, opiates, tranquillizers, barbiturates, actually increase their own metabolism, meaning the more you take, the more and faster you metabolize the drugs, and the more you need. This is a sign of physical dependence, and can be a step towards addiction. Some people get drunk easily, others need more.