Being Fair

I have always thought that if you love someone, truly love them, then you should let them know. But how far does that go? Should you tell someone you love them (or love them still) when they are in a relationship with someone else? This question has been on my mind a lot lately. Is it fair for one person to say 'I finally have things together and can give you what you deserve and need" after years of being gone. Would that be considered selfish? Even if you consider each other soul mates, is it still fair to profess our love and desire for a person if they are with someone else?

Is it the same as cheating if you are trying to cause someone who is in love and happy in a relationship to look at you in a loving relationship manner? Or to put it better, is it the same as cheating to try and gain someone's interest meaning pull them away from their current spouse?