I have never really understood what the driving force for the expansion of the Roman empire was.
The Brits wanted more trade (at least initially) and the Spanish wanted the gold / other resources. What drive the Romans forward?
Start with the history – the city of Rome began its expansion when the last of the Etruscan Kings were overthrown and the Oligarchs in charge looked with lust towards the city of Veii. All the conditions were right for Roman expansionism – a favourable climate enhanced food production translating into a growing population which the city itself could not support therefore expansionism was inevitable. The Greeks were sailing increasingly westwards bringing their ideas and items for trade which the Republic took very quickly to heart.
After subduing their neighbours and rising to the top of the pile, the Romans effectively repeated these steps throughout history. Oh I dare say the sack of Rome by the Gauls also changed the Roman mindset incorporating a “never again” mentality which is why the Punic Wars were a vicious set of campaigns thrown in with the wars against Macedonia in support of their Carthaginian allies.
With the death of Alexander and his heirs splitting his former empire several ways, there was a large power vacuum in the Mediterranean and the Levant which the Romans were the first to take advantage of. The discovery of huge silver deposits encouraged Carthage to look west and the Romans quickly followed. The burgeoning Republic was no longer able to feed itself so grain ships from Egypt had to be secured and the path from Egypt to Rome had to be kept clear of any foreign or pirate interference hence the need for a large army and navy.
But the army and navy had to be kept occupied – an idle professional army in those days are just as likely to overthrow the rulers and the generals and admirals would take charge. The slave-based economy also required a constant and steady source of supply in order to keep the currency buoyant. Rome was in a unique position to take advantage of all these factors, of the weaknesses of her neighbours, the weaknesses of local politics by providing strong leadership and that’s why the Roman Empire was in a position to dominate events for a millennia.