The fall of Rome wasn't something that happened over night. The whole process lasted hundreds of years. Also, all of Rome did not disintegrate. Only the western half of Rome fell; for the eastern half was wealthier and less afflicted by barbarians and civil wars. There is no single reason for Rome's decline. However, it can more or less be broken down into three parts.
The Germanic tribes were responsible for a lot of the pressure on Romans at the time. The sporadic barbarian attacks left border regions destroyed and depopulated. In order to defend themselves they needed a stronger army so higher taxes and labor services were forced upon citizens causing middle and lower class people to hate the government because it had taken so much from them.
Also during this time Peoples spiritual sense began to diminish. Their feeling of individuality and power of intellect became less and less. The urban upper class where the people who basically made the society work began losing interest in public life. The aristocrats hid inside the walls of their estates and did not lift a finger to help the situation. The townspeople were very angry and acted on this by avoiding public services and rarely putting up resistance against barbarian invaders. Many farmers viewed the Germans as liberators. The masses of the roman citizenry eventually gave up even though they outnumbered the barbarians.
The roman government itself contributed to the spiritual malaise through its increasingly autocratic tendencies, which culminated in the regimented rule of Diocletian and Constantine. For many, the state had become the enemy, and its administration was hated and feared more than the Germans.
In the late Roman Empire the quality of Roman soldiers and the legions failed to defend the borders. The army consisted predominately of provincial peasantry.