http://tvtropes.org/pmwiki/pmwiki.php/Main/StalkingIsLove Hi, how are you, I love you and can't live without you... Now that is something that everyone wants to hear, but when do you know when someone is taking the, "I can't live without you" part too far. There isn't a real way to know unless the other person is stalking you. If the person says it because you both know for a fact that you are truly in love and are soul mates, than they are probably ok. But if you are looking out your window and see them in a distance staring straight at you then you need to call for help because that person has taken it to a whole other level... That level is called Stalker Mode!
Don't get me wrong, everyone including myself love hearing "I love you and can't live without you", but once that turns into the other person becoming a stalker than that starts to get weird and creepy even to the extent of where it is scary. If the other person turns into a stalker, you need to get out of that relationship as soon as possible and as carefully as you can because they can turn into a scary person if they see you with your friends or even in another relationship.
Not every person turns into a stalker, don't think that they do because they really don't. Some are actually truly in love with you and find you to be their other half "their soul mate", but you might run into a few who might turn out to be a stalker. Just take the precautions you need to, especially to keep yourself safe. Your safety is extremely important.
Love is a gamble that everyone at some point in their life has to take, sometimes it is good and sometimes it is not good. Whatever the outcome turns out to be, you need to be careful because you never know what can happen (most of the time it is good).