why did the Romans decide to conquer all of Italy?

Question

The Romans decided to conquer all of Italy 

Answer 1

It was probably for national security reasons.  In central Italy, there were Etruscans, who were enemies of Rome.  There were Gauls in northern Italy, who were enemies of Rome.  There were other enemies to the south.  It is likely the Romans decided to conquer all of Italy to prevent attacks by enemy neighbors in the Italian peninsula.