How do I change my ally's attitude (toward me) from defensive/neutral to friendly?

[Wizzington]

Relations do matter under some circumstances where both Defensive and Friendly are valid attitudes and high opinion will weight it towards the latter. One way to encourage them to be friendly is to build trust - attack countries they want stuff from and give them said stuff in the peace.

 

[Wizzington]

Aside from guessing, is there a way to tell what an allied country wants during a war?

Claims and cores, giving them any province that's useful to them will give trust (you get more for stuff they really want tho).