Why do you think a roman sailor would be glad that controlled the entire Mediterranean sea?

He would have control of all the trading routes and ships, which would make him very wealthy. The Mediterranean Sea was the main body of water that trading ships used to sail from place to place to trade with people from other lands, which would make those items worth more in their native lands since they weren't native to that area. For example, spices from China were valuable in Rome since they couldn't be grown there.