What are two reasons the Roman Empire eventually collapsed?

Answer 1

Historians have cited many reasons -- some more rational than others.

http://www.roman-colosseum.info/roman-empire/reason-why-the-roman-empire-fell.htm

What do you think?

Answer 2

http://www.csun.edu/~hcfll004/fallrome.html

Answer 3

1. The Romans had conquered many other countries and failed to fully incorporate those individuals into their culture. The empire became too large and too culturally heterozygous to manage.
2. The Romans failed to address new military challenges and change the way they fought. They were conquered by unorganized Germanic warriors. The Romans continued to fight in organized battalions and were overrun.