Some of the most important lessons in life, are the ones that are the most difficult to learn.  Friendship - true friendship, isn't something that happens every day - and a lot of times, a dysfunctional friendship is easily masked as a "healthy" friendship. 
I have had to learn this lesson one too many times.  But I turn 30 in 2 days (April 20th) and my resolution for starting this new chapter in my life is to be very choosey on who I allow into my life - because while it is easy to let someone in, the scars left behind are not as easily removed.