The only way to deal with, hysterical, over-emotional, mercurial people is to not care too much about what they think or are feeling at any given point. Take them too seriously and you find that your emotions run riot right along with theirs.
You can't take their feelings seriously until they stick for a period of time, then you acknowledge them. They will still probably pass, but you have to deal with them while they are around.
These up-down-up-down people take what they feel very seriously, so you can’t ever openly dismiss them. This does not mean that you don’t care about them, their lives, or that you are not interested. You just don't care about what they feel.