Full Idea
If one has to imagine someone else's pain on the model of one's own, this is none too easy a thing to do: for I have to imagine pain which I do not feel on the model of pain which I do not feel.
Gist of Idea
To imagine another's pain by my own, I must imagine a pain I don't feel, by one I do feel
Source
Ludwig Wittgenstein (Philosophical Investigations [1952], §302)
Book Reference
Wittgenstein,Ludwig: 'Philosophical Investigations', ed/tr. Anscombe,E. [Blackwell 1972], p.101
A Reaction
I just don't feel a deep problem here. Wittgenstein didn't know about mirror neurons, which trigger in me a reaction like the one causing your behaviour.