Enemies suck, but at least your relationship with them is honest. You don't like each other. You don't wish each other well. Whether you're the type of enemies that loathe each others' existences and are constantly at each others' throats or you're totally content to stay as far from each other as possible, it is what it is. There's no trust there, so you don't have to worry about them betraying it. Enemies can't hurt you the way friends can.
The amazingly wonderful things that come with friendship, most importantly love and trust, are precisely the things that make the hurtful things hurt. You confide in your friends. You give them part of you and trust them to cherish it.
There are many ways they can betray that trust. They can expose your deepest secrets or lie to your face. They can do things they know will hurt you or compromise your relationship if you ever find out. There are many ways they can painfully twist your loyalty and devotion until it's mangled beyond repair. Those lines are clear. Friend turns foe.
But to me, the greatest pain they can cause it to hurt themselves and refuse to acknowledge that it affects you too. At least with the other jabs and stabs, it's direct. When someone you care deeply about tells you she is going to take care of herself and then doesn't, it hurts both of you. Not only is she breaking the promise she made, but she's hurting someone you love...and may even play the victim card to make you feel guilty about your anger or pain.
This raises the question: Is she friend or foe? You love and trust her, but she hurts someone you love and trust. How long can you take the cycle before the relationship is too far gone?