Why it is smoking is said to be dangerous to ones health?

The residues (tars and particulates) accumulate in your lungs, degrade their efficiency and cause cancers.

There are plenty of scientific studies proving that smoking damages the lungs. Not everyone gets lung cancer, but it is a factor in setting the process in motion. Other health effects include heart and circulatory issues (stroke, heart attacks) and it may make some conditions (diabetes) worse.

For young people who smoke, you don't see many of them involved in sports that require running. Their lung functions are impaired so that they don't have the ability to breathe as needed for that strenuous activity.