It is necessary to look within and notice how you truly feel. Your body knows the answer to many of the mind's questions, if you would simply let it come up within you and listen.
When you think of a person, feelings come up. It's very general, more of a body sensation than a comprehensible "list". Does your stomach knot up? Do you relax instantly? Perhaps you even get itchy or tired or extremely excited. These things are crucial to your health -- both mental and physical -- because each time you are with this person these emotions and body senses will come up.
So when you've just gotten together with a friend and you say goodbye, and you walk away, what are you left with? Dig deep down (actually, it shouldn't be that far down, but the fact you've never paid attention makes you think it would be) and pin point the area in your body that is screaming for your attention. Is it telling you "Yes! I want more of that! That was so lovely!" or is it telling you "Run. Run for your life because this person is eating you alive!" Two extremes, but two very possible scenarios.
Along with feeling your body comes responding. You must take appropriate action. Go with your GUTS, instinct, with this feeling you've just discovered by listening to yourself. Sure, your mind generates thoughts about people, but your ego will most likely analyze and fight for something that is not always good for you. Now once you've listened to yourself, you've gotta reflect this honesty physically and change something. Maybe this person is healthier in smaller doses. Maybe your girlfriend or spouse is not treating you fairly and it's time to make some changes in your relationship. Maybe you need to start spending some time alone so you know what you want and who you want around you to make you feel a certain way.
First, listen. Then, feeling comes. Now you must act accordingly, be strong-willed and understanding with yourself. It's difficult to make these kinds of life changes. We don't want to hurt those who we feel are hurting us (most likely without their knowing), but in the long run the most loving thing you could do for not only yourself, but for them also, is to let them go.