He has a girlfriend: When you are on a date with a guy, and before leaving, he tells you with an undertone that it was nice seeing you, the chances are high that he has a girlfriend. It is possible that his girlfriend is on vacation or he is in a long-distance relationship.
"Nice knowing you", or most commonly written out as "It was nice knowing you", is a phrase that is generally used when you're not going to be seeing this person anymore and they will be out of your life, so you will not know them anymore.
In this case, the subject is 'to see you' and the complement is 'nice'. It's not 'it' and 'to see you' that are identified as being equal, but 'to see you' and 'nice'. The sentence says that 'to see you = nice', not 'to see you = it'.
1. You Too "You too" means you enjoyed seeing someone and is short for "it was great to see you too." You may remove "it was great to see" in informal situations to show that you also had a good time seeing one of your friends or doing something together. You too. I'm glad we got a chance to do this together.
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc. See also: (it's) nice to see you. it was nice to see you. Hey! (it's) nice to be here. (it's) nice to see you, too. (it was) nice talking to you. nice talking to you.
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it was nice seeing you meaning