When someone loves you then you become exclusive but when its just lust you will feel like just another person being used.
Pure Love versus Lust?, Consider you feel very strongly about helping others, then someone comes along that is in a terrible position in life and you want to make that person feel better, you come to know them and their plights and seek to help them become the most they can be, while this is sympathy it is also a form of 'Pure' love, you came into the situation with compassion and seeking to know the person as they are, you have resolved yourself and accepted that person and wish to bring them happiness, the only issue, do they also feel the same for you?
You are the one who has fallen for them, you have seen them as they are and love them for it, you want to bring them happiness, but many times this is not mutual and the person will simply take advantage, this is where we come to the crossroads between being 'in-love' and loving someone.
I see Lust as a temporary, short term, thing-of-the-moment where emotions are high and relationship is strong.
As oppose to love which is long term relationship where you care for someone so much that you are willing to defend them with your life. You don't care about anything else and the whole point is to be with the person as long as possible.