Some research suggests that unhappiness does not predispose people to illness.

There are distinct questions here:

1. Factual: does being unhappy tend to cause other sorts of harm to you?

2. Moral: is unhappiness something unfortunate that happens to you, or is it wrong on your part?

The research suggests that the answer to (1) is no. That's fortunate, for unhappy people. But whatever the answer to (1) is, it does not mean that being unhappy is blameworthy. If there is no ethical, reliable way to decide to be happy, then your unhappiness is no more your fault than an injury or a birth defect.