Gladden comes from glad + -en. Glad is from Old English glæd ('bright, cheerful'), from Proto-Germanic *gladaz ('shining, smooth, glad'). The suffix is part of the word's derivational shape, marking the final grammatical or semantic role while the base supplies the central meaning. As a verb, it means "to make glad or joyful", so the modern sense keeps the older action or relation visible. Relevant historical forms include Old English glæd and Proto-Germanic *gladaz. Derivative forms include gladdened, gladdening; related words include glad, gladly, gladness.