In what ways does the federal government step on the rights of the states? why?

This is a polemical issue, because in many cases federal government step on the rights of the states. Usually, federal power is used to decide about a national matter, that could be only decide by the highest powers in the Nation, which is utilised in a Federal level and its highest jurisdiction around all Nation.

Several topics demonstrate this like gay marriage, marihuana consumption, guns laws, and so on...

About gay marriage, some states, internally, don't approve it, but now it's a federal right that two people with same gender can marry. On the other hand, we have guns policies. Buying guns is allowed in all states, however, Federal government (not state government), regulates the types of guns that can be purchase by specific people.

the us constitution is a living document, meaning that it can be changed over time as the country changes.

they were byzantines