I think often it's disbelief.
I would love to know what is going on in a guy's mind when he says that. Is it just an expression of appreciation - or is there some underlying expression of commitment - like "'you're a keeper."
I can tell if it's just a line to get sex - those ones are easy to dismiss. I'm not attracted to that kind of guy anyway, so it's not really an issue for me.
To me, as much as it's nice to hear - these words are confusing when they come too early. You may trust the man, and believe him that he is sincere, but it seems to open the door to mixed messages early in the relationship.
Unless it's love-at-first-sight for both of you - like you are soul-mates, then differences in expectations may make those words seem insincere. This can make it take longer to build trust in the relationship.