What affects it? Sometimes it is positive and sometimes it is negative and I do not fully understand what affects it. Does the first dialogue choice you get have any impact on it?
Each civilization has an agenda that you can learn about throughout your game. Civ reactions to you will depend on how much you are in line with their agenda. The "First Impressions" diplomatic modifier lasts a couple dozen turns after you meet a civ and can greatly benefit you to be on the good side of a civ.
For example, an enemy civ may be technophiles and love it that you are first place in technology. If you are lower on the leaderboard, they may start to get hostile and stop once you've reached higher levels of technology.
Your dialogue choice does have an effect since there are modifiers that you can view in the diplomacy window (relationship value) that are added/subtracted each turn. Sending a delegation right away helps by providing you a more positive modifier that will buy you time to find out and satisfy their agendas, if you wish to do so.
That said, sometimes a Civ will still reject your delegation when your starting relationship value is too low. This may happen when your civilization does not fit their agenda.