First of all, in a 1v1, you should probably have diplomatic victory off.
what happens is each player constantly generaterates diplomatic points depending on their research and relations. The exact way is a bit confusing and I dont know how it works. Diplomatic victory says you win if you have certain number of diplomatic points.
You can change the rate at which everyone gains diplomatic points in the game options. I highly recommend not having this set to fastest, normal or slow is probably what you want if you really insist on having this option on.
