what stage of growth is your Tama currently? I know that the younger ones, infant to teen, tend to have a lot more stress than Adults do, for each of my adult stages the amount of stress tapers down considerably, but the youngsters need constant attention.
also, the higher the weight, the higher the stress, I've had to play Sound Bloc constantly until the weight was back to normally because of my bad snack behavior >.>;
Either way, the more that you leave you Tama alone, the more that it's stress level will go up, no matter what you do. I've heard somewhere here that certain characters are more stress prone than others, but at the same time i have no idea if that's true or not.