I'll give it a go
I'll mark things im not certain on with italics
Diplomatic Inclination: might be based on fleet strength (too strong to fight so we'll coooperate)
Racial Inclination: +1 if same race -1 or -2 if others
Adjacent Territory: proximity to their emipire, further away is better
Missions: increase from completing missions max of +3
Mission Rejections: reduction from rejecting their missions
Fleet Strength: large fleets negatively impact this, small fleets positively impact this
Military Actions: things like helping them win battles or not helping
Diplomatic Actions: giving pacts
Resources Given: bonus from donating resources outside of missions
Envoy Bonuses: relationship generated from envoys
Research Bonus: researched diplomacy stuff
AI Relationship Bonus: dont' know
Do they need to complete MY missions?
Do they need to give ME resources?
Yes, just give them simple missions like "kill x ships of player abc" when they are already fighting player abc's ships, instead of the stupid "kill x civillian buildings of playeriveneverevenseen" that they spam you with
If you give them resource missions i think you have to give something back because they just reject if there is nothing in it for them, and don't forget to target yourself 