Many philosophers think that mental states can be reduced to physical states. It seems to me however that properties such as sadness and happiness are adjectives that apply to a person's mental states. It doesn't make any sense to say "this is happy brain tissue" does it?
That reasoning is suspicious, as you can see when you use it in another domain. You might say a certain ice sculpture really isn't just a bunch of H2O molecules, because the sculpture is beautiful; and surely the molecules aren't beautiful. That would be bad reasoning. We know the sculpture just is the molecules (what else could it be?), so we simply have to get used to the idea that a bunch of molecules can be beautiful. Likewise, we might have to get used to the idea that brain tissue is happy, if the reductionist view of mental states is generally well supported by arguments. Admittedly, that sort of talk sounded odd to me too when I first encountered the idea that the mind is the brain, but I can't say it sounds terribly odd any more.