What were two reasons for the decline of the Roman Empire?

1: An influx of barbarian peoples from the Eastern steps, like the Huns, Vandals and Goths, who's methods of warfare were different to what the Romans were used to dealing with. The Romans were very good at fighting set formal Battles, but were not so capable of dealing with skirmishing forces which eventually wore them down.

2: Economics, the Roman Empire was built on the back of cheap slave labour, it was tradition however to free loyal slaves after a lifetime of service or upon the owners death, though not always the case.

Slaves were usually captured in wars, when the Roman Empire ceased to invade other countries the amount of slaves they took was reduced, their economic model went into decline as a result.

3: Incompetence at the highest level of government. The people in charge simply became to comfortable, too self assured of their own superiority. Eventual this hubris cost them an empire.