I often forget small promises to my spouse. Why does it hurt them so much?

Small promises broken again and again erode trust, because your spouse reads it as a sign that they are not important. For you it is a trivial thing forgotten; for them it is proof of whether your word can be relied on. Write down the promises you make, even something as small as buying something on the way home, and set a reminder. If you truly cannot keep it, let them know early and apologise; do not just let it disappear. Keeping small things builds your spouse's confidence in you on big things. See communication & emotions.