Toxic People...

http://stason.org/articles/life/self-growth/Stay-Away-From-These-Four-Types-of-Toxic-People.html

Someone once told me that you should take notice how you feel after you spend time with someone, and if you feel drained and down instead up uplifted and positive, that person is toxic in your life, and you should probably stay the hell away from them... This article is really good at explaining it.