Why is tramadol addicting?

ONLY FOR SOME. Nearly anything that stimulates the 'pleasure center' in the brain can be addicting. Not everyone has the same effects from using tramadol, but those who get a 'high' or 'rush' are at high risk of addiction.
Similar effects. Components of tramadol/ultram chemically react in a limited similar manner as opioids at the cellular level of the body. Examples of opiiods are vicoden, percocet, morphine, heroin, oxycontin. It is possible to get physically dependent and become addicted to tramodol.