"Glad" and "happy" are almost interchangeable as synonyms Somebody can feel happy for no reason, usually a more subdued enduring sense of well-being, satisfaction, and enjoyment. But there has to be something concrete to feel glad about, usually associated with a short-term experience of pleasure, joy, or delight through some immediate cause. You can say that "happy" is "happier" than just "glad". In a business letter expressing pleasure, then you would use "glad", rather than "happy".

"Happy" means feeling good for any reason. "Glad" means you are happy BECAUSE you are grateful or appreciative about something. I am happy when I am eating ice cream. I am glad that I remembered to buy ice cream yesterday so that we have some to eat tonight. It makes me happy when I hear mockingbirds singing in my neighborhood. I am glad that I live in a neighborhood that has mockingbirds.

"happy" is a twice stronger feeling than "glad"

Happy Synonym positive meaning similar to the word "happy." cheerful, glad, content, pleased, delighted, joyful, elated, lighthearted, & thrilled