In the diplamaticbehaviour file, I see the attitude trend was used as a condition, but I don't see where we can get it? is it something we can earn? or some random factors?

I think this is based on the trend for the AI "score" of attitude to you. That is whether they are warm, hostile, or etc.



When you invade their colonies, you might get messages that they don't like you as much because you did, and they are currently "Warm, friendly, neutral, suspicious, hateful" or some other score. You can see the trend as an arrow pointing up, down, or horizontal in the diplomacy screen next to the picture of the alien/ai when you go in to see alliance status and so on.



So, yes it's sort of under your control a bit it's based on your interactions with the AI in game and how friendly you are in their eyes.