Thread already started when re-setting contentView

I have this code on my MainActivity:

@Override
public boolean onTouchEvent(MotionEvent event) {
      float x = (float)(event.getX());
        float y = (float)(event.getY());
    if(y>=screenh/3&&y<(screenh/4)*2)
        setContentView(g);

g is a surfaceview. When I click on this specific place on the screen I want to set the contentview to the surfaceview, but it gives me "thread already started" exception at:

    @Override
        public void surfaceCreated(SurfaceHolder holder) {
                    gameLoopThread.setRunning(true);
                    gameLoopThread.start();
        }

if I set the contentview at onCreate to g, there is no error. Why?