Is my goldfish lonely or sick?

I have a fish tank which has 10 neon tetra fish, 3 guppies and 1 common goldfish. For the past few days, my goldfish has been lying on the bottom of the tank and it doesn't move around much. Is it lonely cos its the only goldfish in the tank or is it sick? If sick, then why are the other fishes not? Is it something to do with their species? If you have any knowledge, please answer.