Romromrom,
I too had this bit of confusion when I first downloaded Diplomacy. You are looking at two numbers: 1) How much "your people" like "their people"; and 2) How much "their people" like "your people". When you send your envoys, give resources, complete missions, and help them out in combat: You improve how their people view your people, This makes them receptive to Pacts and whatnot initiated by you. Good times!
Getting them to the point where they can offer pacts ad whatnot is interesting. To improve how much "Your People" like Their People" is a little more difficult. They have to do things that your "People" like. The easiest way that I have found is to give them missions that they are already doing. If you want to improve how "Your People" like the Red Player, and the Red Player is fighting the Blue Player, give the red player a "destroy ships" mission with a decent reward. (I like to use: Destroy 5 Ships. Reward: 1000 Credits, 200 Mineral, 200 Crystal). Experiment with this, and you will see the number in your box improving.
The other way that you can improve your "Peoples'" perspective is to fight in joint actions against a mutual enemy. (You can usually join an attack with an AI that is above 0.0, against one that is below, and expect good results as long as you target ONLY the enemy fleet).
Other than this, I have trouble getting the AI to send me envoys, or make other gestures of Good will... 
Anyone else?
Ged