adjective : having an excess of sentiment or sensibility : having or expressing strong feelings of love, sadness, etc., in a way that may seem foolish or excessive
Next morning Fred showed me one of the crumpled flowers in his vest pocket, and looked very sentimental.
— Louisa May Alcott, Little Women, 1868-9
Being sentimental was originally a good thing. Sentimental meant "marked or governed by feeling, sensibility, or emotional idealism." If you were sentimental you had a kind of emotional intelligence, in modern terms. The word comes, of course, from sentiment, which has the same origin—Latin sentire, meaning "to feel,"—as consent and sensible.