There are many retro slow song which are really nice and makes to happy. One of the best is "La La Means I Love You" by The Delfonics
That is a really nice song. The Delfonics released that song in 1968. As a small child I have memories of seeing my parents slow dance to that song in the early 1970s. I seen the Delfonics perform that song along with other songs when they were doing an oldies concert in the mid 1990s. It was an oldies but goodies tour. The line up included the Delfonics, the Dramatics, the Stylistics, and some other old school groups. All of the groups at that concert sounded great.