Love is an emotion.
In 2269, Harry Mudd tried to sell love via means of him love potion crystals. Although the effects of the potion were temporarily effective, once they wore off, the resulting effect made the individual's emotions for each other to hate. (TAS: "Mudd's Passion")
Captain Montgomery Scott compares starships to females, stating: "You don't ever love a woman quite the way you did that first one." (TNG: "Relics")