The Knight of the Laughing Tree and the first signs of love
Rhaegar Targaryen was the son of Aerys II Targaryen, known through the kingdom as the Mad King. Per George R.R. Martin's source material, just before Robert’s Rebellion, a tournament was held at the castle of Harrenhal with Aerys, Rhaegar, Rhaegar’s wife Elia Martell, the Starks (Brandon, Ned, Lyanna and Benjen), the Lannisters, the Baratheons, the Daynes, and many other noble houses across Westeros all in attendance. Rumor has it that Aerys only attended out of suspicion that Rhaegar organized the assembly of high lords to conspire against the throne and remove the Mad King from power.
The tourney lasted ten days, and the realm’s finest warriors participated in jousts, axe-throwing and archery contests, horse races, and more. One jousting match occurred between a mysterious knight in mismatched armor, who won against three other knights in a tremendous victory. This mystery knight was dubbed "The Knight of the Laughing Tree," named for the laughing weirwood tree emblazoned on their armor.
The identity of the Knight of the Laughing Tree was hotly contested -- yes, even Westeros has Game of Thrones theorists. King Aerys was convinced it was an enemy put forth to mock him, and he ordered his son to find the knight and unmask him. Rhaegar was only able to find the knight’s shield, which was abandoned by a tree.
Rhaegar eventually won the final joust and was named champion of the tourney. In a surprising turn of events, he rode straight past his wife Elia and placed a crown of blue winter roses on the lap of a young Lyanna Stark -- who was betrothed to Robert Baratheon at the time -- which made her the "queen of love and beauty," a traditional title given to the loved one of a tourney winner to whom he dedicates his victory. This shocked the crowd, angering Aerys, the Starks, and Robert.
What does this have to do with the Knight of the Laughing Tree? Context clues point to Lyanna being the mystery knight. Some suspect that Rhaegar didn’t just find her shield, but discovered her true identity and was moved by and attracted to her bravery, which is why he made her the queen of love and beauty. Either way, the Great Tourney of Harrenhal is where Rhaegar and Lyanna first met, and for many, marks the beginning of Robert’s Rebellion.