When in doubt, say thank you. No one is going to be upset about being appreciated too much, as long as it's genuine.
Gratitude is often left implicit. It's easy to convince ourselves that these feelings go without saying. But, that's just asking for trouble. When you have something to say, you should say it. It's unreasonable to expect people to read your mind.
People who express appreciation are more likely to be perceived as kind and empathetic. Studies show that expressing appreciation leads to happier marriages. Couples who explicitly expressed their gratitude were more likely to be able to power through stumbling blocks (e.g. money related issues).
Intuitively this makes sense. Feeling appreciated on a bad day can be a huge energy boost. It seems worth erring on the side of being overly thankful and potentially making someone's day.