To be honest, I'm usually lying when I say, "Nice to meet you."  There are the occasional instances when I finally meet someone I've been hearing a lot about or when I meet the significant other of a close friend or relative.  But most other times, I'm merely subscribing to a set of social standards.  I shake your hand, I say, "It was nice meeting you," and we go our separate ways.  It's usually not that it was bad to meet you.  I can only think of a few times when I wish I had never met a person.  It's mainly just that it's not "nice".  If anything, it's neutral.  I'd rather say, "We met today," and part ways.  But I obviously can't say this because it's not socially acceptable.  The thing is, we'll probably never see each other again.  We'll almost definitely never communicate.  And even if we happened to cross paths at some distance point in the future, neither of us would remember the other person's name, leaving us with the choice of awkwardly avoiding and ignoring each other or doing the whole "Haven't I met you before?  Weren't you at that thing?  Aren't you friends with my friend?"  So if you ever hear me say, "It's nice to meet you," please understand that it's not literally "nice" to meet you; I'm just fulfilling my social obligations by reading from a memorized cue card.  No offense.  You probably do it too.