There are an abundance of romantic and sentimental songs, happy, sad, romantic love, parental love and sibling love. Sentimental and romantic tends to make great use of the elements of musical harmony and dynamics (dynamics are the loudness or softness of a composition).
Certain words are more emotionally charged than others in both prose and poetry. When setting music to text more stressful words or words that imply love and passion may be accompanied by chords of higher intensity (those which sound more passionate or intense). This intensifies the emotional meaning of the words for the singer and the listener.
Dynamic levels too, play a part as they are a natural indicator of emotional mood. Soft dynamics, are usually associated with tranquillity, repose, calmness and sensuality associated with love songs, lullabies, wistful and melancholy songs. Loud dynamics are associated with vigour, turmoil, conflict and valour, for example marches, fanfares, and triumphal music tend to be loud.

Holmes, Robyn. The collectors book of sheet music covers, Canberra : National Library of Australia, 2001