You get aggressive expansion when you declare war on someone without a casus belli, when you take provinces in a peace deal or when you annex a vassal. If you either have a core or a claim on the provinces you are taking you'll get less aggressive expansion. Pay attention to the peace deal screen when in a war and it will tell you how much aggressive expansion that peace deal is causing you. It's worth noting that the aggressive expansion the game shows you is the base value, the actual aggressive expansion you get with each country is modified by their opinion of you, their opinion of the country you took provinces from and their distance from you. So, if for example, you take provinces from Aragon, Genoa won't care much, but Aragon's allies will, as will any country that named you a rival. Countries far away, like Poland, won't care either.
About improving Relations with Aragon, forget it. Diplomacy with a country that named you a rival is futile. However if you become more powerful militarily than Aragon (either because you have more provinces and a larger army or because they lost a great part of their army in a war) they might change their attitude from rivalry to threatened and they diplomacy will become effective again.
