Welcome Mavie,
Depression hits both JWs and non-JWs. There could be a hormonal problem with your wife. It could be something terrible that happened to your wife in her past. These past "life experiences" lead many to the Truth. The Truth helps for a while, but then those demons arise. Like a glass of milk, if we don't deal with our past....it festers and hurts to deal with later.
I applaud you for helping your wife. The best "ministry" or "field service" we can give is to our families. The elders have forgotten that. By caring for your wife in her time of need, you are setting a great example for your children and others near you. We are to treat our wives as Christ treated us. Christ would not have left a sick person to "take care" of themselves.
I am a 3rd generation ex-JW. I've watched depression set in for the past two generations as my relatives got older and faced death. They never thought 1975 would happen, nor 2000. Depression & mental institutions at the end. While depression hits both JWs and non-JWs, I can personally attest that it does hit JWs, especially the older ones. Very good on the advanced education.
Skeeter1