Too much conflict on Game of Thrones is built upon secrets and lies, as the Season 7 finale emphasized. The Stark and Lannister conflict was built upon a lie told by Petyr Baelish through Lysa Arryn, and Robert Baratheon's Rebellion that landed him on the Iron Throne was built upon a lie about Rhaegar Targaryen and Lyanna Stark. What happened to Rhaegar on Game of Thrones? The Last Dragon finally appeared on the HBO series, but his story didn't end with that wedding.
Despite baring a squeamish resemblance to Daenerys' brother Viserys Targaryen, the prince was by most accounts a nice guy. In Season 5, Barristan Selmy recalled that Rhaegar preferred singing to killing, and used to disguise himself as a minstrel, roam the streets, spread music, and give his profits to charity. But, Rhaegar was involved in a tangled web of romantic drama. Cersei Lannister was supposed to marry him, and both she and her father Tywin seemed keen on the idea. Instead, he was married to Elia Martell, but he was in love with another — Lyanna Stark.
The only hint that Rhaegar did not kidnap Lyanna, but ran off with her, was the Tourney of Harrenhal — after which he gave his favor to the Stark girl instead of his own wife, Elia Martell. When he an Lyanna ran off together, all seven hells broke loose. Rickard and Brandon Stark rode North and were imprisoned and killed by the Mad King Aerys. That prompted Ned and Robert to rebel against the Targaryen dynasty.
Rhaegar was not involved in any of this fighting until the very end. He left his pregnant wife guarded at the Tower of Joy, and died during the Rebellion in the Battle of the Trident. It was Robert Baratheon, who was betrothed to Lyanna Stark at the time, who took him down in a single combat duel, scattering the rubies from Rhaegar's armor into the river.
The future King slaughtered the prince with a battle hammer — just like the weapon that Robert's bastard son Gendry has taken to swinging around. The former blacksmith thinks that his and Jon's fathers were friends, but as Jon's father is Rhaegar instead of Ned, he couldn't be more wrong.
All of this could have been avoided if Rhaegar and Lyanna had been allowed to be open about their affair. Is Game of Thrones doomed to repeat this history, now that Daenerys and Jon have begun a scandalous romantic relationship? History likes to repeat itself, and his father's path could mean Jon Snow will end up in a similar way. It's just one of the many things that Season 8 will have to address.