When you live with someone day in and day out (regardless of whether it's a family member or a roommate or a significant other), it's easy to take that person for granted. The nice things they do for you or your home or others become commonplace. They become expected.
One of the things I've learned from living with Matt is how good it feels to be thanked for these things (and to thank others for these things). For example, Matt certainly doesn't need to thank me for doing my half of the chores. It's my responsibility! But when he thanks me for it, he's acknowledging that he noticed my effort. That my contribution is important to him. He's communicating, "Hey, I'm glad you help maintain a system of equity in our relationship. I appreciate your reliability."
Plus, saying thank you helps increase the number of positive interactions in your relationship.
John Gottman--a sort of marriage mathematician--argues that a solid relationship has five positive interactions for every one negative interaction (which Malcolm Gladwell featured in his book Blink). Saying thank you to show appreciation for various things your partner does seems like one way to increase the positive interactions.
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