In his interview with Mustapha Mond, the World Controller in Aldous Huxley's Brave New World, John tells him that he does not want contentment over truth. He tells the Controller,
Getting rid of everything unpleasant instead of learning to put up with it. [Here he quotes from Hamlet]Whether 'tis better in the mind to suffer the slings and arrows of outrageous fortune, or to take arms against a sea of troubles and by opposing end them...But you don't do either. Neither suffer nor oppose. You just abolish the slings and arrows. It's too easy.....What you need is something with tears for a change. Nothing cost enough here.
An anachronism in the New World, John the Savage is the last truly civilized human being. He does not want comfort at the expense of his humanity:
I want God, I want poetry, I wnt real danger, I wnt freedom, I want goodness. I want sin.
When John does take soma, it is out of his terrible desperation; like his mother Linda, he has despaired of life. Putting himself into a soma-induced coma, John escapes artificially when he has been unable to physically escape the ogling of the Deltas and others from a world entirely abnormal and foreign to him. His destruction is not just the destruction of an individual; it is the destruction of the individual to Huxley's view of utopia.
This is not a very clear question -- are you asking why soma is bad in general or are you asking why it is not right for John in particular to take it?
To most of the people in the society, soma is good. But John did not grow up in that society. He does not like soma because he thinks it takes away his human emotions. The people of the society like having soma to take away their emotions. John, on the other hand, wants to have feelings. That is why he wants to read Shakespeare, for example -- he wants to experience all kinds of human emotion, even if they are painful.
So John refuses to take soma because he does not want to be emotionless like all the people of the civilization. Instead, he wants to be a full human being with a full range of emotions.