Love and infatuation

This topic came up as an aside in one of the forum threads recently. I also asked some of my friends to define it. In neither case did I find the answers satisfactory. For example one said, and this was typical, "Healthy love differs from obsession in that it evolves towards something deeper and more meaningful whereas the other becomes a sickness.

Personally I find these answers very unsatisfactory as words like "deeper", "meaningful" and "sickness" have no clear meaning to someone enjoying euphoric love. :) By the time you discover one of you has a sickness it is usually far too late, the damage is done.

So the question boils down to, how do you know if his / her / your feelings are healthy and positive (or indeed honest).