Well I'm not sure this is quite all of them, but it's a fair start
Positive Modifiers
1)"Years of peace have strengthened our relations" (from being at peace for several turns)
2)Open borders policy,
3)"We help our brothers and sisters of the faith" (sharing same religion)
4)"We appreciate the years you have supplied us with resources" (have traded them resources for several turns)
5)"Your trades with us have been fair and forthright" (you have agreed to several deals with them, or plain gifted them stuff)
6)"You gave us tribute" (you gave them something when they demanded it)
7)"You gave us help" (you gave them something when they asked for it)
8)"Our mutual military struggle binds us together" (You went to war as the result of a defensive pact, or you bribed them to go to war)
9)"You helped us in wartime" (you agreed to go to war at their request)
10)"You shared your technological discoveries with us" (you gave them a tech which only you possesed)
11)"You have chosen your civics wisely" (you have the other civ's favourite civic)
12)"We appreciate the years you have supplied us with resources" (You've been selling/giving them resources for some time)
13)"Our Defensive Pact proves we are close friends" (you have a defensive pact)
14)"You accepted our favourite civic"
15)"You accepted our state religion"
16)"You agreed to stop trading with our worst enemy"
Negative Modifiers
1)"Our close borders spark tensions" (You share a significant length of border with them)
2)"You have fallen under the sway of a heathen religion" (You have a different state religion to them)
3)"You declared war on us" (self explanatory)
4)"You declared war on a friend" (you attacked a civ they were at least pleased with)
5)"You refused to give us tribute" (refused a demand)
6)"You refused to help us" (refused a request)
7)"You refused to help us in wartime" (refused a request to declare war on a civ attacking the other civ)
8)"You refused to stop trading with our worst enemy" (you refused to cancel all deals with whichever civ they like least when demanded)
9)"You refused to convert to our beliefs" (wouldn't convert when asked)
10)"You refused to accept our favourite civic" (wouldn't change to their favourite civic when asked)
11)"You stopped trading with us" (you cancelled all deals with them at the request of another civ)
12)"You brought in a war ally against us" (you bribed someone to attack them)
13)"You organized a trade embargo against us" (you signed embargos against them with another civ)
14)"You razed a city" (you burnt one of their cities in wartime)
15)"You razed a holy city" (you burnt the holy city of their religion)
16)"You nuked us" (increases 2 points for every nuke)
17)"You made an arrogant demand" (you demanded something from them)
18)"This war spoils our relationship" (you are currently at war with this civ)
19)"We are upset you have formed defensive pacts with our rivals" (You have a defensive pact with their worst enemy)
20)"You have traded with our worst enemies" (You are either trading, or have recently traded with their worst enemy, this includes open borders)
21)"Your spy was caught causing trouble"
22)"You nuked our friend"
There are one or two others listed in the XML files, but they seem to be either duplicates, or do not actually appear in game. For instance you receive no diplomatic penalty specifically for cancelling open borders, even though it is listed as a penalty in the XML.
