Origins
Pleasant is a word about surfaces — etymologically, about smoothness. It descends from Old French plaisant, the present participle of plaisir ('to please'), from Latin placēre — 'to be acceptable, to be liked'. The deeper root may be Proto-Indo-European *pleh₂-k-, meaning 'flat' or 'broad', suggesting that what pleases is what goes down smoothly, without friction.
The Latin placēre generated a rich family in English, each member capturing a different shade of agreeability. Please is the direct verb. Pleasure is the state. Placid means 'undisturbed' — pleased to the point of calm. Complacent means 'pleased with oneself' (con- + placēre). Placate means 'to make someone pleased' — to smooth things over.
Latin Roots
The most curious relative is placebo. In Latin, placēbō means 'I shall please', and it was the first word of the vespers for the dead: Placēbō Domino in regiōne vīvōrum ('I shall please the Lord in the land of the living'). Medieval professional mourners — hired to sing at funerals — were mocked as 'singing placebos', performing grief to please the family rather than from genuine sorrow. By 1785, doctors had adopted the term for treatments designed to please the patient rather than cure the illness.
Pleasant entered English in the 14th century and has remained remarkably stable. Unlike many words that shift from positive to negative over time, pleasant has kept its gentle warmth for seven hundred years.