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Thanks, guys, some good explanations here, and all seem to be pointing toward the same thing. I think what I relate to best is OldGeezer's statement that love comprises a number of things, in different ratios.
I wonder if people who say they love someone, but are not in love with that person, actually analyse their emotion in that way - that is, they might realise there are a lot of feelings of friendship present, but not a lot of loving commitment - or maybe it is based on a "gut feeling".
I'm afraid it is a bit of a mystery to me, but maybe I over-think the issue.
Thanks again, all who answered.