Most kids like to take things apart to see how they work.
For me, it combines the sciences of physics, chemistry, and anatomy into a meaningful fascination that none of these has on its own. If we know how things work and how they break, we can know how to fix them again. Hence, medicine is a branch of physiology. Physiologists also are fascinated with the way organisms have evolved to cope with particularly harsh environments with slight modifications to their DNA and enzyme systems. It is interesting to contemplate how primitive organisms which do not have highly evolved organ systems have managed to adapt to such environments as deserts, hot springs, fresh water, cold, heat, nutritionally poor food supply, etc.