the word 'sweeten' is a transparent english formation from the old english adjective 'swēte', which itself descends from the proto-germanic 'swōtuz', related to the idea of pleasantness. the verb-forming suffix '-en' (from old english '-nian') was productively attached to adjectives to create causative verbs, meaning 'to make (adjective)'. the literal sense of making something taste sweet is primary, while the figurative sense of making a deal more agreeable is a logical semantic extension based on the positive connotations of sweetness.

Structure: sweet + en sweet (from old english 'swēte', meaning pleasant to the taste, not sour or bitter; functions as the core adjective root). en (from old english '-nian', a verb-forming suffix; functions to convert the adjective into a verb meaning 'to make or become').