I'll echo this:
AmytheTemperamental said:
It falls under the disorder of normal reactions while around other humans.
It's a perfectly valid statement and one should be concerned if they showed no reaction to things or people appearing in front of them.
If you feel like tearing your eyes out in the process of seeing a particular somebody, that is unhealthy of course. What's to be done? I'd say learn to let go of your negative fixation by focusing on something else or avoid seeing that person altogether. The only disorder that comes close in my mind is borderline personality disorder which encompasses "unstable emotions". It seems like a matter of emotional control at least. But to be honest, you don't want to be diagnosed with this just for shits and giggles.